Partly I think it's because I watched Episode 6 for YEARS as a kid and never knew it was part of a trilogy. When I finally saw eps 4 and 5, I was underwhelmed.
I will say this though. I think Ep 7 has me seriously rethinking my like of eps 1-3. (well...more like 2 and 3) It's really highlighted for me just how not-great they are. They're very flawed, of course, but there was still a thread of the story that i enjoyed. But now that ep 7 has come out, and they can't even boast of better technology, they are just falling completely flat.