In my actually professional opinion....

The first Star Wars was great because Lucas took about ten or twelve years to write it. I normally take twenty years to write an original work and most writers will tell you the same thing.

And since this was a commercial project with a sequel in mind, and there had to be a sequel in mind when this was pitched in order to get funding, the sequel also had some thought put into it.

And then, when the first three episodes were greenlighted, some story had to be fleshed out and fleshed out quick. This leaves little time for story or character development because now the studio is on your back and screaming for content. The film has been presold in overseas markets, distribution deals are in place and now the Suits are screaming "Where is the finished film!"

So you get a hack job. Considering the restraints Lucas was working under the first three films came off better than anyone had a right to expect.

And.... Lucas is an experimental film maker. Episodes four and five were his experiments with the only constraints being his budget and his thoughts. The rest of them have been commercial products with commercial constraints.

And it is easy to see the difference.