I don't have any proof to provide (and if I did, it would all be anecdotal) but I've heard rumours that Microsoft are planning on breathing life into codeplex and have an exciting/significant set of changes in the pipeline (which should improve on some of the things that make using DVCS painful).

Considering Team Foundation Service, project Kudu/Git deploy on Azure etc. it makes sense to think that Microsoft are making a play to seriously take on GitHub and Atlassian, so that they're the one provider that owns an entire software ecosystem, end to end.

Either way, I don't think that the EF team would have had a choice on where to host it...