If your old market is tanking, you don't have much choice. PCs still provide income for Microsoft, but there are few or no opportunities for investment oriented around growth, and Microsoft ultimately wants to be seen as a still growing company and not as HP. So given that MS makes money on PCs, but can't grow this or invest in any interesting way, it makes total sense that their attention would drift elsewhere to markets where they can grow profit.

We aren't very far apart here. The PC market exists, MS will still make money on it, but the future is somewhere else and we need to be there, especially since we can leverage and invest the money we still make from the PC market today.