People are choosing it because it's what you choose.
Interestingly, since Ogre3D went for Mercurial - most popular open source 3D rendering engine currently - I see more and more projects using Mercurial. Not only directly related projects, but also others.
Only 3 years ago, there wouldn't be any question about it: Git/GitHub.
Now there actually is, due to Mercurial picking up steam.
Choice is great.
Still, the web development community is still 98% Git/GitHub - so maybe it's natural for Yii to become part of that club.
Since you're going for Git/GitHub, I really hope that it turns out to be as rewarding as you wish it will be.
It takes some time before people start realizing that they've got a new set of tools.
But let's hope that we see a lot of forks and pull requests.
Pulling from a clone is so much better and quicker than dealing with patches.