What about Apple? They're clearly working on WebGL support for Safari. They have at least equal insight into the whole stack, considering the ship the GPU drivers with the OS.
Then there's XNA in Silverlight. If they believe in the security of that, why not build WebGL on top of it? Probably because they're in direct competition with one another, and Microsoft wants Silverlight to win.
Apple has announced that, for now, WebGL on iOS will only be available to WebKit when it's displaying advertisements through iAd (where Apple controls which ads are distributed), and not to web pages generally.
I would think that security in this regard is far easier for Apple than it is for Microsoft. By virtue of their closed hardware, Apple has a very limited set of graphics cards for OS X to support. I'd imagine that this makes graphics drivers a lot easier to police and keep secure.
Let's face it, AMD, nVidia and Intel have >99% of the PC GPU market, and 100% of the Mac market. The drivers only change from one chip generation to the next, and the Mac has had chips from all recent generations from all 3 manufacturers. The total number of drivers is therefore identical. I really don't think there's much in it. See also: Silverlight's XNA.
Silverlight's integration with the host browser, DOM and Javascript is pretty good.
I'm pretty sure you could build a WebGL API on top of Silverlight by yourself. It would suck, but it is possible.
That's why I always thought Silverlight is pretty cool as a technology, as compared to Flash. Among other things, a Silverlight plugin could also allow you to serve OGG Theora videos to users.
Then there's XNA in Silverlight. If they believe in the security of that, why not build WebGL on top of it? Probably because they're in direct competition with one another, and Microsoft wants Silverlight to win.