Heroes of Might & Magic III: Horn of the Abyss

But it doesn't necessarily have to be the dwarf race specifically; as already mentioned, the dominant race being dwarves is one thing, and the race itself is another.

For example, Inferno has a demon race, but there isn't such a fixation here. Imps, Gog, Cerberus, Demons. Variety. The same goes for the other standard cities (Castle and Conflux are exceptions).

So, if it's allowed for humans, why not for everyone else?

If you reason like that, you'd have to create a city for each unit (figuratively speaking).