Tons of people prefer the Microsoft environment.

Neither Google nor Apple have offered many of these people what they want in a dev environment. Apple's required an odd language up until very recently, and even the C# language you can write to doesn't do many of the things they're used to doing C# in.

Android's development resources are capable, but Eclipse is a far cry from Visual Studio...

I assume tons of people will flock to the Win7 Phones unless they are just horrible once they get going.

Many game companies can target this platform much more easily than the iPhone/Android handsets as it also works off XNA.