jabird,
It's like many a main hub airport, there are few routes that can be, profitably, served by the larger aircraft unless the thru connections are offered and many of these feeder routes will only be profitable if served by the smaller aircraft types.
You mention FlyBE but, so often, LoCo's only operate at inconvenient (wrong) times of day for both business and leisure travellers alike ..... by example if, as I did, one is travelling LON/BUD/ODS one would like to depart LON at a reasonable time of morning to arrive in ODS at a reasonable time of afternoon/evening and vice versa.
MALEV had a few sizes of B737, they obviously needed them to, well atleast make the effort, operate route(s) cost effectively, Ryanair and Wizzair both, each, operate just the one type of aircraft(s) restricting their route viability.