Elves live on the surface and compete with humans for land and resources. The dwarves, however, lived underground and have a stranglehold on resources that the humans want, most notably lyrium which only dwarves can safely mine. Dwarves are thus deemed useful while elves are seen as competion.
It's that, but its also the fact that dwarves had been highly accommodating to humanity compared to the elves. Dwarves supply humans with lyrium, and sell it the way humans have asked them to sell it, be it to the imperium of old or the chantry currently, the dwarves do a good job at keeping the resource flowing, and keeping the quality from dipping.
Elves don't trade with humans, and more to the point elves can commonly be heard of preying upon innocent merchants passing through their lands. Dwarves may look down on humans, but they don't go out of their way to hamper the humans economic or regional stability.
So yeah, its a case of the dwarves treating the humans well and getting treated well in return.