Nokia had reached the end of the road on Symbian and needed a new direction.
This is a risky thing to do and going with Windows Phone gave Nokia the backing a very large and still influential company. Going with anything else would have meant going in alone. This includes Android which would be going alone into an already crowded market.
The current situation isn't great for Nokia, but they are in a deal that has the potential to help both companies, Nokia with short term financial help and Microsoft with a strong vendor to create showcase phones and the distribution network to get them into consumers hands.
Microsoft killed RIM a few years ago, the monster is just taking a while to succumb to it's injuries. Microsoft shipped an update to Exchange that enabled wireless syncing and push to WM5 devices, and licensed to client to Apple for iPhone and some of the Android OEMs, then finally to Google. This killed the need for BES. The carriers opened up real IP0 connectivity for iPhone and Android as well as WM eliminating the need for the BB internet service and email, and the extra fees payed to RIM. That part of the business isn't dead yet, and the BB is still popular in a number of markets, but RIM is unlikely to succeed in the US with a tohc device
While that contributed to the fall of RIM, what really did them in was that they didn't have a real operating system that could be leveraged to properly compete with Apple and Android.
This is a risky thing to do and going with Windows Phone gave Nokia the backing a very large and still influential company. Going with anything else would have meant going in alone. This includes Android which would be going alone into an already crowded market.
The current situation isn't great for Nokia, but they are in a deal that has the potential to help both companies, Nokia with short term financial help and Microsoft with a strong vendor to create showcase phones and the distribution network to get them into consumers hands.
Contrast this to RIM who tried to go in alone.
EDIT: fixed typo