Harrison Ford: the best and most intense action star of all time. We're talking Star Wars, Indiana Jones, The Fugitive, Air Force One, etc. Spent the last dozen years discussing his hatred for Star Wars, phoning in his performances, and being generally unpleasant.
Mark Hammil: Known for Luke and the Joker, and only those two things. Repeatedly dragged back to reprise those roles much to his own exhaustion.
Carrie Fisher: it would feel like bullying to go into detail. Let's just say very very tired, not a lot of lingering love for Star Wars, or lingering memory of anything.
The point is that obviously the three main leads aren't really going to enjoy being dragged back in, so why do it at all?