A dedicated service provider such as Swissport can be more viable and flexible as it can obtain better utilisation and therefore productivity from its workforce. Their bag chuckers can go from one customer’s aircraft to another and then to another with little or no gaps. This is how costs are held down - you don’t requires as many people. On the other hand, if the same people worked for one carrier, they may well be idle for sometime between tasks, depending in that carrier's schedules.
The gravy train has stopped for the TWU. And not before time!