Why Not for Original XBox?

Because Xbox is Huuuuuge.

Yeah, original Xbox is old news these days. If you're dying to play, your best bet is to snag a PS2 and the game from Ebay or somewhere else that you can get it remotely cheap. Microsoft probably wants to entice players to get a 360 if they want to play GH2.

You probably already know this but Microsoft has stopped virtually all support for the original Xbox. There will be no new games coming out for it at all. Personally I would rather go with a Sony PS3 that will support and make games for its system for the next 10 years instead of replacing an xbox every 3-4 years due to lack of support. If you want to go inexpensive and fun, get a nintendo wii, Guitar Hero will be coming out for it as well.

No it is not, and it probably never will be. Aside from the PS2 and the XBox 360, the only other console that is confirmed to be getting a Guitar Hero game is the Wii, although a PS3 version is likely as well.