For the project I work on, we moved code hosting to github years ago, but still have our downloads hosted by SF.net, and haven't even thought of what to do about mailing lists and tickets. We'll get around to moving it all, eventually, but for small teams working on a project with so many users, so many files, so many mailing lists, so many tickets, and just a lot of pieces that make up the "project", it's a lot of work, and the gains may not be immediately felt.
I don't know where the NetHack team is in their migration, but it is apparent they're working on it. I'm not gonna criticize them for not having it done already.
And, of course, there is no guarantee that github won't turn evil, someday, too.