When I tried both at that time hg was just really slow so I just adopted git for all my personal projects because it was fast and a lot better than cvs. I imagine others were the same.

I went with bzr mainly because it had an easy way to plugin "revision" into my documents in a way I could understand and monotonously increment.

hg was slow though I don't know how bzr compared as I was using it pretty light-weight.