Dwarves are unpopular among players because they are short and they cannot be mages.
I am a 5'5" guy and am practically invisible. I know that is not too short but it is short. I personally find it a good thing, it means I can just go around do whatever I want because I am not noticed very much.
Because of this I tend to actually play dwarves a lot. Primarily as rogues or as an infiltrator-engineer hybrid. I like Dwarven tech and their short height allows them to make mischief without raising too much attention.
In most fantasy games, I usually play as Elven mages and as Dwarven rogues.
I actually dislike playing as humans or qunaris because they are so tall and I find it hard to be them for roleplaying purposes. Also I am a human in real life so it is boring to be one in a fantasy as well.