Lots of good thoughts here, but let's not discount Alma's appeal that it would be good for their daughters.
I know some folks feel Ennis uses his daughters as an excuse, but I think he does love his little girls and probably is susceptible to the argument that it would be good for them to be closer to other kids to play with (though I note that we only see them playing with each other--on that swing set--though let's also note that Ennis and Alma do set up a swing set for the girls), and not be raised, as he was, on some lonely old ranch.