It is the eternal quest for hearts and minds. Developers. Developers. Developers. Microsoft have always been aggressive about getting their stack in schools here. Hook them young. To be fair other companies try and do the same. It is good to see Microsoft on the Pi. Diversity is important. It is better that they get behind the Pi than them going off and promoting a more closed alternative that would only boot a signed Microsoft image.

As more and more things become connected and automated, why would they not want to extend their platform to those devices?

The architecture of the Pi is common enough that is probably serves as a proof-of-concept for commercial vendors who want to build programmable products that would be well-supported by operating systems and device drivers (by piggy-backing on the work done for the Pi).

There's actually a very good reason. Windows XP has been officially unsupported by Microsoft for about two years now unless you are willing to pay them a quarter million dollars a year. Often the computers spec'd for those systems have pitiful amounts of CPU power and memory (i.e. much less than necessary to run Windows 10), and the cost of a re-spec for PCs in certain critical systems is basically the cost of developing a new platform (not to mention that some of them can't fit a microATX-sized PC or have low power requirements) makes it potentially cost-effective to use SBCs, especially a popular one which someone like Newark/Element14 can customize for you and potentially support.

If kids start playing with Windows early on then they may grow up with a more positive view of MS.

The Raspberry PI has a substantial commercial market. That plus as a way to retain interest in Windows, their support makes pretty good sense.

Microsoft lost the smartphone game cause they came too late with a mature product. Supporting Pink ensures they stay in the IoT game from the start.

Education.

More hands on experience for the students with all the available hardware extensions.

Mindshare?