Mercurial went a long way down the "never modify history!" road (i.e. never rebase). Turns out that attitude, while theoretically pure, isn't very pragmatic. There are lots of valid reasons for rebasing.

Hg eventually came around but, for me, the damage had been done. It's the second biggest reason I went with git.

(The biggest, of course, is you use git if you want to interact with kernel devs. But lots of kernel devs hated git in 2005/2006. Mercurial had a reasonably big window of opportunity... all imo, fwiw)