I think it’s more that the OT depended on off-screen development. Putting the films together requires acknowledgment of that development being skipped over.

Prequels, especially AotC/RotS, take place during a war with the romance tying it together and scenes that kinda show the characters being reunited after a gap in time. This makes a 3-in-1 simpler. The sequel trilogy similarly has two films back-to-back and the next film has a “recap” scene of sorts when Poe and Finn return to Ajan Kloss at the start of the film.

The OT doesn’t have the same kind of connective tissue, it’s much more designed to imply gaps between episodes.