One reason for this is increased system stability, gained by moving things out of kernel space. A laudable aim, but not really an important one given that I'd think most sound drivers are pretty stable, especially compared to their GPU counterparts. Also, with MS moving things like HTTP handling into the kernel, this reason rings less true.
Another reason, and presumably the more important one, is that Microsoft are altering the audio stream to make DRM more effective, as referenced here.
So, many of your existing games are going to run more slowly, sound worse, or perhaps not run at all, so that MS can work with people like the MPAA to protect their interests, not yours.
Developers can opt for OpenAL which still goes direct to the hardware, bypassing this problem. I have no idea how well supported it is, or how awkward it is to program with it. It wouldn't surprise me if it ended up being software emulated on all non-Creative sound cards. Does anybody know any more on this?