It's got nothing to do with simplicity of the plot, but they way it's told. The PT hides the plot too much IMHO. The OT takes the viewer along. One can definately argue that the PT has a better plotline, but I think they way it's been told is very poor.
I think Lucas should have checked out Alfred Hitchcock a bit more. Hidding plot points is not clever if you're not good at it. Hiding Sidious's identity was useless and made everything fuzzy in TPM and AOTC (and tehrefor unexciting), hiding the people behind the assasination attempts of Padme was not wise, as if it was such a big deal to find out who did. And when they did, we didn't care because Dooku was an unknown to us. The storytelling was faulty in the PT, more faulty than in the OT for sure.
A kid gets that, he sees all the flashy things in the PT and likes it a lot. But that he gets sucked into an exciting story where he wants to know how it ends, how the characters will do... that always shows. And that is where the PT fails miserably.