Probably one of the main reasons for lack of performance is the existence of a big de-rate on the engines, increasing life cycle/decreasing fuel burn etc etc.===money arguement. So if the basic cost of the flight goes up, pax figures need to be on the increase aswell, the equation to break even goes up.
Instead of 80 pax necessary to break even you need 100 for example.
It's all accountancy and money drives this sector