1. Nokia had a Maps business. Obviously, Elop wanted to see it on the chosen platform. 2. Nokia had a host of apps that they wanted to see on the chosen platform. 3. Google refused to allow modifications to their MADA. 4. Meego/Maemo were not viable/not ready. 5. Microsoft's offer came with an infusion of cash.
(Bloomberg article: http://www.bloomberg.com/bw/magazine/content/11_24/b42320567...)