[acdha]

The difference is that you're talking about something which is MUCH smaller: someone can port the CDM to a new platform in far less time than it'd take for a complex platform like Flash or Silverlight because all you're dealing with is the DRM, not things like networking, display, sound, hardware accelerated video decoding, OpenGL, etc. Both Flash and Silverlight are to a first approximation as complicated at the entire web stack and almost none of that functionality is needed for a simple video player.

Yes, it sucks that it's proprietary but since consumers don't show any sign of refusing to buy DRMed content it makes sense to restrict the footprint as much as possible rather than letting the requirement for DRM drive your entire platform.