Because Gamespot is in bed with Microsoft. Nothing more, nothing less. And the only bit that's OS dependant is the crappy ass installer.
Anyone running UT2k4 on non-XP Windows, or Linux, or OS X can download from worthplaying and unzip it to get the exact same content (the WP file appears to be a zip, not an exe, so even though they say "all Windows" it's really "all platforms").
The point, however, remains -- it is a slimy thing to do, and there's going to be some people who will think they can't get it at all because they don't run XP.
According to the Steam hardware survey, about 6 percent of people are using something other than XP. Support for Windows 98 was stopped ages ago, so the next logical step would be to buy XP and download Linux for free (it is a free OS still?). I'm pretty sure there is no real reason to install Linux, other than to stick it to the man.