Maybe it's just me, but to me the barrier to entry is much smaller if all I have to do is clone a github repository, tweak code to solve my problem, push, and then send a pull request. Using SVN, I can't really contribute without becoming a committer or jumping through hoops. With SVN, the workflow would be to checkout anonymously, tweak code to solve my problem, create a patch file, locate the issue tracker, create an account on the issue tracker, post a bug report, and then attach the patch file.
You would still do the same with GIT, HUT, etc. It's the way the workflow works.