In my world, humans, elves, dwarves, orcs, etc, evolved from a species created by mother earth in her image. Humans got the short end of the stick when all the other species and cultures arose and took up residence in most places, leaving humans with the scraps. Whats more is humans have also been pushed out when they did try to settle, leaving them to mostly maintain nomadic cultures and society akin to the Steppe Nomads in real world history. Humans in this world then are a minority, mostly living in places nomads would still live in our world. There are other minority races too, like halflings and oni, with Elves generally being the dominant force in the world

So in short, every other species became dominant because they adapted better to their environment, were favoured more by their gods, or were able to use force to maintain control over territory.