GitHub and BitBucket are proprietary web applications. This makes them unfit for hosting Emacs.
Also, the pull request system is not very good, IMO. It encourages bad practices like having to use git push --force to submit a new patch set without opening a brand new pull request (I suppose you could delete the remote branch and recreate it each time). And then there's the merge commit that it adds if you merge via Github, rather than rebasing to master and doing a fast-forward for a clean history. As ugly as it is, I much prefer submitting a patch set to a mailing list.
Also, the pull request system is not very good, IMO. It encourages bad practices like having to use git push --force to submit a new patch set without opening a brand new pull request (I suppose you could delete the remote branch and recreate it each time). And then there's the merge commit that it adds if you merge via Github, rather than rebasing to master and doing a fast-forward for a clean history. As ugly as it is, I much prefer submitting a patch set to a mailing list.