Microsoft looks to substantiate the Silverlight Media Player in the age of Flash and HTML 5. Brad Becker, Microsoft director of product management has said that, Silverlight is not designed to replace HTML5. The closed-source player, he contends, lets you build "premium" experiences.
Becker was also quoted saying, "Silverlight delivers "consistency" and "timing" for those looking to offer online video, games, and consumer, business, and enterprise applications."
Becker claims that since there is no set date for HTML 5 and that fact that Microsoft has released 4 builds of Silverlight in the time it has taken HTML 5 to be put together that developers should get on board and start using Silverlight. That said there are some HTML 5 media players out there. Microsoft's stance is that HTML 5 is not ready to develop the premium experience users are expecting.
Microsoft claims consistency,availability and the premium experience makes Silverlight the more attractive choice.