Why not? I know the reasons three years ago (and most of them were either permissions or performance), but I have been assured multiple times that this is no longer the case at all.
 

Because the community on Github is significantly larger.

At some point, this devolves into bikeshedding. Twisted devs would prefer Launchpad, but many people hate Launchpad with a passion. Between Github and Bitbucket, as you've said yourself, Bitbucket is playing feature catch-up (whether they're doing that successfully or not is something I'm willing to skip discussing). I don't think features are the thing to differentiate the two on (even though I think Github wins because of polish). It's network effects. Github has more following, so it's more interesting.

The thing is, it's not so much a vote. This is a do-ocracy. The breaks were "I'll move it to Github, *or* someone stops me", not "I'll move it to Github, or Bitbucket, or whatever else you folks think is a good idea". If people are going to try and stop me from moving it to Github, I'm not going to move it to Bitbucket or anything else. It's just going to stay Trac/SVN.