Vendor lockin. Because there is a lot more involved that accessing files in a .git directory. All those CI systems and workflow plugins and whatnots such as travis-ci work with github, not git. And all the inflight issues and pull requests etc. Migrating or giving up the features you use has a cost (not just in dollars), and if github has done their sums right most customers will realize they are better off staying where they are.
Travis CI is pretty minor player (google search 10M > hits), and I would say 3rd party service integration is exactly vendor lockin.
Imho this makes sense, because users who cared about pricing already moved on to cheaper alternatives and/or not used github to begin with. I agree, mass migration or phasing out github is unlikely because of the associated costs, however with all the other great services around new users might think twice where to sign up.
