I mean, GitHub supports svn clients but it’s git all the way down. Granted I haven’t used svn on GitHub but I doubt it’s treated as a first class citizen. This is really just a stepping stone to migrating your VCS to git.
This is the wrong question.
The right question is why did git need github in the first place (while svn didn't)
svn had forges (sourceforge, rubyforge, etc.) that fulfilled many of the same things that GitHub does.