- Nokia itself isn't a very healthy company any more
- Their CEO originally comes from MS (thus, the Windows Phone platform)
- Microsoft sees with Apple and Samsung that it's probably a good idea to have the hardware and the software.
At the end of the day, this is just a logical consequence. One could have foreseen that when Elop took over the CEO position. Well maybe. With a little "omg, when it's Microsoft, they're probably buy it" at least.