It's because while Xbox 360 was released the same year World of warcraft was, the Xbox was built to play games at a specific level of intensity and game companies never had to worry about anything below that. Warcraft on the other hand, had to make sure that computers older than 2005 could play it. Essentially, Blizzard prides themselves on World of Warcraft being able to be played on a toaster. If you have a decent graphics card, with everything at ultra settings it doesn't look that bad.