read the related Ms article "WebGL considered harmful".
there its stated that they wont include it in BROWSER beacause it is considered harmful. so silverlight is not include either. even if it is exactly the same as WebGL
But they were pointing out that what WebGL does would never pass their security reviews. This isn't even the IE team from my understanding but the security team at Mircosoft making this statement. I'm sure they work on multiple projects.
