This has me wondering, what Microsoft wants GitHub for? It's not like they can't use it without owning it, and I doubt they want to improve it as it is. Probably they want control before integrating it with their products.
They have a clear long-term strategy of owning professional social networks, both within and across companies.
That way they can sell you access to your professional network, and your employer access to info you don’t even realize exists about you, and also sell it insights into its own org structure.
GitHub would just be another piece of the puzzle.