I get your assertions about Episode I, but I would have to say because I think he knew he was going to some pretty dark places in Episode III. On top of that, the tone of Episode I, I believe, is that way for another reason and that's to mask how deceptive Episode I really is in the context of the Saga.
I mean, if you strip away everything in Episode I, it's really about Palpatine's machinations to get to the highest office in the Republic. That's serious stuff. And the celebration is a false one because they technically won but didn't really win in the grande scheme of things.