American and Delta both paired up with Mesa at one point, and swore Mesa off as a result. They know UAL "has their hands full" with Mesa at the moment. Mesa is probably doing more financial harm to United by hanging around than going away, from a competitive standpoint.

Either United is expecting some kind of comeback Cinderella Story from Mesa, or United is keeping them afloat to use them as leverage against the other regionals down the road. Those are two of my guesses.