It's the money, honey. Jack would've divorced Lureen in a flash - for Ennis. However, because Ennis wouldn't and couldn't commit to him, Jack had no reason to give up his comfortable lifestyle w. Lureen. For what? He apparently enjoyed some of the creature comforts. Have you seen their house? Notice the paunch he develops as he gets older. Ennis once remarked that "Jack's not the restaurant type." I doubt that was the case after he married Lureen. Notice his late model trucks. Notice his fashionable clothes, like his L.L. Bean, quilted, down parka in their last scene together (I recognize it from the catalog).. Contrast that w. Ennis's faded clothes, beat-up truck and shabby trailer.
Jake stayed w. Lureen the way some wives stay w. their abusive husbands. Economics. If Ennis wouldn't commit to him, where would Jack go and what would he do if he left Lureen? For one, he'd be out of his lucrative sales position.