Rovio provided a bit of insight (at 140 characters at a time) in subsequent tweets:
@wmpoweruser always dangerous to make serious analysis based on 140 characters. Main reason is not WP7 itself, it’s just different.
@wmpoweruser can’t just take c++ and open gl code and put it on WP7. Would be different if we only did WP7 from scratch and nothing else.
For perspective: iPhone software is written using Objective C. Android software is generally developed using Java but an NDK is available for C and C++ development for critical components. Windows Phone 7 applications are developed using Silverlight, XNA, and .NET Compact Framework 4 using supported programming languages.