It's because mercurial is legacy, like darcs, bzr, monotone, and the other zoo of "git-alikes" that were big in 2008. Now it's just like MacOS Classic in 2000: there are a few die-hard zealots who claim that the interface is better in some indescribable way, and everyone else has moved on.
Mercurial is very important for Facebook, Mozilla, Google, and others. The disparaging "legacy" monicker might be a good way to feel justified to not have to learn about it, but it's not dying software that is no longer getting updates.
Speaking as someone that works with Mercurial on daily basis, and also makes/sells software that supports Mercurial. I can tell you it's far from beeing legacy.
