TLJ wasn’t really a middle story, though. It was a downer ending. After that there was nowhere to go without a generational time skip and a completely new story that would be inappropriate for a trilogy. There was no big antagonist anymore, there was barely any protagonist left, and every dangling plot thread was ruthlessly cut short.

Contrast it with ESB and you see with that you while have a bittersweet end to the movie, you do not have an ending of the story. Lucas even left room to bring back Han who he just sort of killed.