Rock Band and Guitar Hero might not be all of music gaming, but they are the two biggest and the most similar. They are big communal gaming affairs with a laundry list of licensed rock music and weekly downloadable content.
kraken's initial post wasn't "WHICH IS TEH BIGGEST n BEST MUSIC GAME EVAR?!?" It was "which do you prefer from these two?" because these two are directly comparable. If you don't like RB or GH, then abstain from voting. If you prefer another music game, go talk about it elsewhere.
This is for Guitar Hero vs. Rock Band, not GH/RB vs. Every Other Music Game