They launch the API with such poor quality samples that the API can't evolve without triggering loads of backwards compatibility issues (and so it never evolves).
Microsoft APIs often don't evolve - they're replaced or forgotten.
The trap is blindly following all the new technologies. Yes, I do find myself liking new features in different releases of the .Net framework, but on the other hand :
Does anyone use Silverlight?
If I'm writing a desktop application, I've yet to find a compelling reason to do it in WPF rather than Winforms.