Moving to Github is simply developer friendly.
Don't misunderstand me. Microsoft's products compete with open source software in the marketplace for developer mindshare and I am not suggesting that this is not a business decision. Being on Github helps the company iterate quickly rather than on the multiyear cycles that were standard when software came in a box.
Will Microsoft buy Github?
Only if Github is selling.
And there was value for Microsoft. There isn't amazing technology, there. There's a huge liability for litigation hosting just anyone's content that comes with Microsoft size pockets, and Git, being distributed would allow an angry community to set up their own Son of Github if Microsoft took control and acted improperly.
Incidentally, WiX has been on SourceForge since 2006. There's been nary a rumor of a deal there.