I think the lackluster performances of AOTC and TPM (as far as public opinion goes) really hurt ROTS's chances... People sour on a franchise and I think a lot soured on Star Wars after 1999 passed us by.
No, I think it has to do with the fact the ROTS isn't that better than it's prequel predicessors (as far a public opinion goes.)
Actually, I think what everyone has to realize is that Star Wars has s built in audience. Unless the films were God Awful, they would earn $300 miliion plus easy per film if they kept making them, and it would be roughly the same audience. What'll keep it going is getting John Q Publics butt's in the seats, and that's done with strong word of mouth. I know ROTS is getting favorable reviews and is hailed the best of the prequels, but that's not saying much if you're not a Star Wars fan and if word of mouth isn't that strong, which it's not for ROTS.