The divorce clearly shifted the balance. Before it both had wives, families, jobs - after it Ennis had lost his wife, his children were largely out of his reach, except for once a month, and he'd never been committed to one job;indeed in the early days he's just given up on one, seen Jack, then gone back to a different one. But he knew that Jack wasn't committed to Lureeen and never had been - the time after the first reunion told him that. Jack might have told himself that Ennis wasn't committed to Alm in the sens eof loving her, but felt he had to stay for the sake of the girls, though to believe that he'd have to be pretty good at seeing only what he wanted to.
Once the divorce was over, Ennis had no reason not to leave Riverton and go wherever with Jack, and incidentally it was by this time 1975 and things must have changed to some degree even in the society Ennis was so afraid of. So Jack was forced to confront the fact that Ennis had probably been using Alma and the girls as an excuse but even once they were out of the picture was never going to commit to Jack on a permanent basis.
Every meeting between then and the final one must have been colored by that knowledge - Jack settled for what he could get for another eight years, but it must have had a corrosive effect on him.