One of the main aspects of Windows Mobile / Phone 7 is the full control Microsoft intends to have on the user experience and on the software.
That means that manufacturers are expected to innovate in design but have no room in customizing and improving the User Xperience. Manufacturers have worked for a long time with 3rd parties to improve the UI, the software, add some missing features.
There is a whole ecosystem which has developed around Windows Mobile devices, even though the biggest manufacturers like HTC, having the resource, mostly developed by themselves. These developers made for manufacturers many different kind of applications: UI, messaging, Social Networks, camera, Media players, etc.
With the full Microsoft controlled User Experience of Windows Phone 7, these partners see their future with Windows Phone 7 very gloomy. That’s why so many of them have switched to Android.
Furthermore the frenetic pace of Google with 7 (!) Android releases in 18 months is exhausting manufacturers development teams. To such extend that some devices are still launched in 1.6. That’s where 3rd partiers have a great part to play. If they manage to follow this rate of innovation on their field of expertise they will become ‘must have’ piece of software for manufacturers. In that case, the main threat may come from Google itself which might integrate their features in the core of next Android release.