Yeah, these things are mostly true (apart from git, which I think has the best possible interface for a VCS), but I don't understand why did you suddenly switch from a question about a decision by a particular company to a question of wide adoption of technology by a community.
And your second paragraph misses the point: it's not about one of the factors, it's about balance between them. In this particular case, the amount of users who prefer XMPP doesn't seem to be big enough to counter the pain of supporting it.
Shame mercurial didn't catch on. I started learning DVCS' with mercurial, back when they were still a new concept, and git wasn't very good (this is an understatement, git was horrible back then).
What I really liked was the very friendly and usable command line interface, out of the box shortcuts (which I now emulate in my .gitconfig), clear help pages. It was a bit slower than git for enormous projects but it worked wonderfully for me.
Git didn't have any of those things either except "lightweight branches". It wasn't until 2006 or so that git grew a reasonable commit command. Before then, people relied on tools such as cogito in order to get work done.
And your second paragraph misses the point: it's not about one of the factors, it's about balance between them. In this particular case, the amount of users who prefer XMPP doesn't seem to be big enough to counter the pain of supporting it.