yeah, I don't get how such a disappointment could score so high either. $$ is my #1 theory. My #2 theory is that it was at the start of a gen with very crappy launch titles, and it was one of the biggest most hyped things out first this gen, so it made people more excited and biased for it.
It's BECAUSE GTAIV was such a massive disappointment. That was just a big open world with nothing to do. Yeah, they had the easter eggs and stuff, but no shops or decent minigames. GTA V is "supposedly" going to remedy all that, but we'll wait and see.