I should also mention GH3 wasn't very well-received because of the sharp difficulty spike. Rock Band then came out and completely made Guitar Hero irrelevant.
There's still two kinds of people who like Guitar Hero:
1) Casuals that don't know about Rock Band
2) Score Hero kids who just want to be challenged
Rock Band is one of my favorite series around, but it's not quite the best game to play if you JUST want to play guitar since it focuses more on the band aspects than having a particularly challenging career mode.
And how quickly we forget that Guitar Hero 3 sold close to 9 million units so far, which absolutely shatters Rock Band. Most gamers "in the know" moved on to Rock Band, but the majority of the world has not.