Because it doesn't support CSS Animations, which I use for insertion detection. I might be able to finagle it for IE9 with Mutation events, but in their DOM Level 3 state, they destroy performance.

IE10 supports CSS Animations and will also be the first IE version to support HTML5 Windows apps, those are the two most significant reasons it is the current minimum version for IE. If IE9 is the thing people really need, I can try to work it in, we'll see what folks say ;)