Because III sucked ass. Well, it didn't really, but compared to the first two (a high standard, to be sure) it did.
Not true -- Lucas had the entire story (including the prequels) pretty much already written and/or conceptualized by the time Star Wars was filmed. He knew that in order to do the entire story justice he'd have to do it in installments -- but what he DIDN'T know was whether there would be a market for the rest of the story. I mean, if SW had bombed in 1977, it WOULD have been a standalone. Happily for geeks and nerds everywhere, it didn't bomb.