It's a bogus technical reason. Chrome and Firefox show that GPU acceleration can work on XP:

http://hacks.mozilla.org/2010/09/hardware-acceleration/

http://blog.chromium.org/2010/09/unleashing-gpu-acceleration...

If Microsoft wanted to support GPU acceleration through an alternate rendering path like two other browsers are doing on XP, they would have done so.

I find it more likely to conclude that they just didn't want to support XP and developed a browser GPU acceleration engine that wouldn't support it as a result of this high-level decision.