Why are humans the dominant race in Scarterra?

A common criticism of modern fantasy is that in a fantasy world with all sorts of fantastic beings with amazing abilities, longer lifespans, or both, how and why are humans, who don't have amazing powers, be the dominant race?   In Scarterra I answer this question and not just with the meta answer "Because human dominated worlds are more relateable!"  

Humans are backed by the Nine

    For one ongoing example is that human genetics are persistent. With few exceptions, only humans can make half-breeds that aren't sterile and half-humans tend to revert back to full humans, even with controlled breeding to try to resist it.   Kingdom of Swynfaredia was founded by dozens of half-human half dragons who intermarried but when two half-dragons have a child, due to the so called "Leaky Bucket" causes human genes to assert itself so the child of two half-dragons is going to be at best 45% dragon. Then generation would be 40% dragon, give or take, so the so-called Swynfaredian Dragonbloods look mostly human with only their aptitude for sorcery separating them from non-dragonblooded humans and even their aptitude for sorcery is slowly fading away with each generation.   The same thing happens with half-satyrs and half-elves, they tend to revert back to humans pretty quickly.   Many believe the "Leaky Bucket" metaphysical law was deliberately crafted by the Nine to give humans an edge, at least when it comes to breeding.  

The Dragon's Path does not make Scarterrans indestructible

    Well, maybe, but Scarterra doesn't have anything like "hit points" or "cinematic damage", so a dwarf who has walked the dragon's path for a hundred fifty years may be very skilled, but he's not going to be more durable against damage than a young dwarf. He's still mortal as it were and can be killed with a couple lucky sword blows from a nobody who got lucky.   Such an amazing dwarf may hopelessly outclass any human, but he's not going to be able to fight a hundred humans and eventually someone is going to get a lucky shot.   Besides, due to psychological limits, longer lived races are not going to be able to walk the dragon's path indefinitely because it will sap their long-term health, even if they are lucky enough to survive all the dangerous things they face. It is also noteworthy that free-willed undead who theoretically have unlimited time windows to grow stronger invariably grow insane.   A lot of humanoid races do not have drastically different life spans from humans. Satyrs are just a little bit stronger than humans and have slightly longer life spans. Theoretically, over time satyrs could outcompete humans, but satyrs preference for breeding with non-saytrs slows their population growth rate, so they are not going to have the numbers to challenge humanity for dominance even if they wanted to.   Tengku and kalazotz have useful abilities but is debateable if they are stronger than humans on the whole. Also, they age faster than humans, so they aren't likely to beat humans by walking the Dragon's Path.  

What about dragons!

  Scarterran dragons are not invincible, but they are the most powerful mortals on Scarterra. Even though they are outnumbered by more than ten thousand-to-one, if the dragons somehow united, they could take over Scarterra. The problem is they will never unite.   The First Age came to an end because the ancient dragon kings and queens could not share Scarterra without fighting. Even though there are much fewer dragons now, the dragons are not in a better position to unify. Its a running joke in Scarterra that dragons barely get along with each other long enough to mate and propagate the species. Any cooperation beyond this is very limited in scope and duration.   Scarterra has other intelligent nonhumanoid monsters that greatly outclass humans, and they can certainly cause problems for humans in a localized areas but they aren't able to displace humans from the face of Scarterra altogether. Ocumati, like dragons have the raw power that could thereotically let them challenge humanity, but the lack the unity among their own kind.   lack the intra-species unity to come together and organize against humans. Others, like giants just aren't clever enough to outwit human armies consistently. Chimeras don't have opposable thumbs. Every non-humanoid mortal race has some kind of limiting factor.  

How did the rise of humans actually play out

  At the dawn of the Third Age, all the surviving mortals of the Second Age were still reeling from their losses. The scattered bands of survivors had to phsyically find each other while the first humans started out knowing where their kin were.     The advantages bestowed on humans by the Nine would be enough to keep humans from being wiped out or utterly dominated, but it would not necessarily explain why they are they are a majority of the Scarterran population and not just one player among many. Intially, humans were were more unified than their Second Age competitors.   The era was rough on everyone, humans and non-humans alike but the non-humans from earlier ages had to purge Infernalists and Cannibal mutants from their midst and the early humans didn't have to worry about this as much (at least at first). Paranoia and mistrust probably caused more damage than the cannibals and infernalists as a lot of innocent people were killed on false accusations.   If the gnomes, kobolds, goblins, elves, tengku, satyrs and dwarves were able to work together better they might have been able to hem in the expanse of the new humans because they had a first mover advantage against the humans but they often didn't trust each other. Since goblins had been slaves for most of the Second Age, they hated and feared non-goblins and attempted to murder or flee from most non-goblins they saw. Gnomes carved a niche for themselves quickly as a valued minority in the lands of others but satyrs and tengku struggled to do likewsie due to old predjudices.   The ancient elves and dwarves were the largest and most organized groups of Second Age survivors and their leaders opted for strategies of fortificaiton and isolation, building impregnable sronghold but giving humans a free rein to expand elsewhere mostly uncontested. Kobolds, ocumati and the surivivng dragons also tended towards isolation even if they weren't actively hostile to the other races. Humans bred faster than most non-humans with slightly shorter gestation periods and much faster periods of maturation to go from child to adult, so the humanx could spread out surprisngly fast.   Eventually, history repeated itself, the humans spread out and after settling the most fertile and productive lands available to them, they started warring on each other much like the elves the Second Age and dragons of the the First Age did before them. Unfortunately this period of human disunity coinciding with the three major elf nations finally consolidating and stabilizing themselves.   During the Red Era, the grey elves wanted to rein in the foolish and short-sighted humans...for their own good of course. They conquered many vassal states where they kept humans as second class citizens, "temporarily".   Rather than colonize or conquer the humans outright, the dark elves opted to establish tributory states and extort nearby human tribes for treasure and slave in exchange for temporary reprieves from fierce military reprisals.   The wood elves took "Get off my lawn you darn kids" approach to international diplomacy to violent extremes and they began claiming more and more forests as their exclusive hunting grounds.   The elves collectively had created a great propaganda campaign where they established themselves as mysterious and all powerful super beings that humans had no chance of resisting. Humans were numerically dominant but they were politically dominated by elite elven minorities. This all changed when the three elf nations finally rediscovered each other. All three elf nations essentially believed that their culture was the last and only real surviving enclave of Second Age culture and power. They were vaguely aware of the other two group existed, but they assumed these enclaves were smaller and more primitive than they actual were.   The dark elves told the wood elves, "join us or die" and the wood elves utterly crushed the dark elf army sent to pacify them. Then a very similar thing happened when the grey elves more diplomatically told the wood elves "join us or suffer" and the wood elves crushed the grey elf army sent against them. The grey elves and dark elves didn't learn their lesson and they fought many skirmishes with each other.   Humans watched this all unfold and it showed them that elves were not invinclble. This fostered greater poitical unity among humans and gave them the courage to throw off the shackles of elven oppression and they pushed the elves back from their lands inch by inch forcing the elves to withdraw back to their original fortified homelands. Whether or not the Nine put a finger on the scale of this conflict is still a subject for debate for theolgians and historians.   Millenia of peace and the emergence of greater threats like Vladimir the Conqueror helped smooth out most lingering hostilities between humans and elves, or at least humans and non-dark elves. While the elf nations still compete with their human neighbors for influene and territory to an extant, even the most crazy meglamaniac elf rulers have no illusions that they could displace humanity as the de facto dominant race of Scarterra. No race other than elves even came close to be in a position to try.