I think the Prequels and Phantom Menace in particular ended up with a significantly more juvenile tone than the OT because George Lucas (self-admittedly) isn’t good at writing dialogue, and since he didn’t have as much help writing and polishing the Prequels as he had with the OT, he resorted to mostly slap-stick humor because it’s just much easier to write.

I mean, in A New Hope, C3PO has some comedic lines that at least required some decent writing skills, like “Don’t you call me a mindless philosopher, you overweight glob of grease!”, or (scolding R2) “Just you reconsider playing that message for him! No, I don’t think he likes you at all. No, I don’t like you either.”