Github also does not have side-by-side diffs, and lacks hg support. I do wish Bitbucket would make a gist ripoff though.

Neither is fork or pull request something that you can't do easily without Bitbucket and Github, but people still use them mainly because those two sites make it easier for them to fork a project and keep the fork up to date. I guess it's all about convenience, and time will tell if people will use it.