The number seems to be misreported as someone else mentioned here, but still, you'd be surprised how lazy people can be when it comes to (large) OS updates. There's an enormous amount of friction if what you use already works. This probably doesn't apply to gamers since you
have to upgrade anyway if you want the lastest and greatest, but everybody else seems pretty happy to continue using XP; I see it everyday in ATMs, in medical equipment, in cash registers, in libraries, etc etc.
I honestly have no idea what's going to take for XP usage to drop substantially. I can only wonder whether this will lead to an IE6-like problem in the future.