To give a personal, subjective point of view of why I switched from hg to git:
- hg was horribly slow compared to git;
- I love the branchs model used in git to let several persons work on different parts of the same project, and I could never find a satisfactory equivalent using hg idiomatisms.
It is true that hg has a far better UI in general, but Magit fixes this problem for me.