It's all down to yield. Seat cost per mile.
The avarage cost in and out of Orlando is low. Compared to New York, Chicago, even Miami. Airlines can't afford to fly the cheaper routes. So they leave it to the low cost airlines. Unfortunately the US has no 'long haul' low cost airlines. So Virgin, Monarch etc get the business.
Years ago we had airlines like 'Peoples's Express', and 'Brannif', and 'National' that thought they could afford to fly the low yield routes. Look what happened to them!
The US airline industry is in a mess right now. The guys that used to rule are all in finantial trouble. People like 'Southwest' and 'Air Tran'and 'JetBlue', who are all making money, don't have the equipment to do the long haul stuff.