Silverlight allows .Net developers to target a Rich web interface from Visual Studio. If you program in windows, you'll naturally choose Silverlight. If you want to develop using open standards, you'll choose html 5. Flash will be gone.
Why do you assume programming in windows == microsoft affinity. Im a windows man and am openly anti silverlight. I am actually amazed its survived this long...
Reasons?
Okay, so maybe don't fuck silverlight and give us <video> to attack flash from both ends? And a pony?