I'm not looking to change simply for the sake of changing. Before Sigil I have never used Mercurial. I am not well versed with it and I have spend 50% of my time fighting with Mercurial. Charles (the person contributing bug fixes) has had the same experience.
My personal preference and favorit SCM is Bazaar. If I could I would switch to it in an instant. However, Google Code does not support Bazaar. Google Code is a good platform and I like it a lot. I think that Google Code does everything except for SCM right. If it supported Bazaar it would be perfect. That said I have no plans to move away from Google Code to Launchpad.
Since I can't use Bazaar I'm left with Git. Git works well enough and I'm more familiar with it than Mercurial. One reason I'm thinking of switching is, Git is very popular. Using an SCM someone isn't familiar with will prevent them from submitting patches. I'm hoping that by moving to Git it will make Charles and my lives easier. I'm also hoping it will encourage more people to hack on Sigil.