Because Flash is not good enough. Flash video playback has an incredibly poor performance, especially with h264 or HD videos, even on average machines or Macs.

Because Flash is not good enough. Flash video playback has an incredibly poor performance, especially with h264 or HD videos, even on average machines or Macs. Silverlight video playback is simply a lot smoother. I'm not sure if it's their codecs or somewhere else in their rendering pipeline, but ultimately it doesn't really matter. What matters is that my users can play back the videos on my site smoothly even on their shitty commodity Dell boxes. That being said, I'd love to switch to HTML5. If only all the browsers settled on the same codec.