Cubase 6 doesn't support XP. That doesn't say it won't run on XP. Steinberg just hasn't put any effort and costs in an outdated OS and therefor cannot and will not guarantee a perfect performance on XP. How far should a software developer have to go to offer backwards compatibility?
Microsoft is now at Windows 7. Before that we had Vista. Before that it was XP. Every new version is designed (so they tell us) to offer more usability and better use of the newest hardware. When you develop software that runs on said OS, you want to take advantage of everything feature you can. Imagine you'd have to take Windows 3.11 into consideration when developing new software...
So it's not necessarily a "shutout scheme" but rather a decision based on cost efficiency and outdated technique.
(BTW: I'm running XP on my DAW and see no need to change until my DAW and BiaB stop working on XP. I hope they have Windows 8 or 9 by then