Great question! I think the primary reason is the originals (4-6) were written/edited/produced by other people, while 1-3 were solely managed by Lucas. No one stands up to him now that he's established and revered.

There's a great review of Star Wars Episodes 1 & 2 on YouTube that are absolutely worth the time to watch:

http://www.youtube.com/user/RedLetterMedia#p/u/19/FxKtZmQgxr...

http://www.youtube.com/user/RedLetterMedia#p/u/0/CfBhi6qqFLA