So... what gives? Why the homogeneity? It makes Rivellon a far less interesting world to explore, IMHO.
Two words: budget constraints.
I don't buy that for a second. They ended up having WAY more money to develop DOS than they thought they'd have, and they did find some time to create a unique creature model for Kirill.
They already have the models for Imps and Orcs in the game, they just don't use them for much of anything. An Elf model would literally require less than 1 minute of fiddling with human model geometry. Same with Dwarves--the basic human model could be repurposed very easily.
The only race that might take more than an hour to create a template for would be the Lizards.
But even then, they've still got high-quality 3D models of all 7 races working with the same engine as DOS from Dragon Commander. IT's just a matter of simplifying the geometry--and most 3D drawing tools I've seen have functions to do that automatically.
This isn't a matter of Larian not having the time to add diversity, or not having the budget--it's a matter of them consciously choosing NOT to have any species diversity.
Hell, they don't even bother to mention half of the races in dialog/lore.