In general, that's true -- but I would submit it's because they've all fallen into this game of heavy hub-to-LGA frequency with smaller jets, as though it's this huge competitive advantage to the business traveler, and as a result their fleets have been largely configured for that sort of thing.

Remember that one of the intended purposes for the 764 at Delta was to serve LGA. They went so far as to work with the Port Authority to strengthen the over-water runway segments to handle the weight.

If there were incentives to use bigger planes, the carriers would plan accordingly. As it is now, there is no incentive. So, we get hourly service from LGA to hubs, but the schedule has to be padded by 45 minutes because of the inevitable delays of trying to schedule that many flights. So the guy who thinks he's lucked out by catching one of the hourly flights an hour earlier than he was originally scheduled when his meeting finishes early, ends up sitting in the taxi line and consuming that saved time. And if weather moves in, it creates chaos and nobody gets anywhere for hours.