Originally Posted by Blumie

Although I posted 30 minutes after grahampros posted the above, this really is the right answer. It's generally more expensive to fly BOS-LGA than it is to fly BOS/JFK-LAX, at least in Y. All of the money is made in the sale of premium fares, and it would be extremely difficult for US to win over those customers, even if it did upgrade its premium product. There is tremendous brand loyalty among those passengers.