Some of the architecture is still part of modern Windows so I don't think they would even consider it. Plus that's millions upon millions of lines of code that must be vetted to prevent trade secrets from leaking out. And I imagine there are other vulnerabilities in newer versions of Windows that would be readily found by examining the older cousin's source.
Another reason why they use it is because XP came out before WGA (http://en.wikipedia.org/wiki/Windows_Genuine_Advantage). This made pirating newer versions a bit more inconvenient (but certainly not enough to stop piracy all the way to Win8, I'm sure). So that leaves the more easily pirated pre-WGA XP
Another reason why they use it is because XP came out before WGA (http://en.wikipedia.org/wiki/Windows_Genuine_Advantage). This made pirating newer versions a bit more inconvenient (but certainly not enough to stop piracy all the way to Win8, I'm sure). So that leaves the more easily pirated pre-WGA XP