I think that when trying to operate the STN hub, AB simply did not appreciate that because the UK is not part of Schengen, and insists instead on full immigration checks on EU arrivals, this type of operation is just too difficult to manage efficiently.
Last year some friends of ours nearly missed their connection from NUE, onward to GLA because of a combination of UK Immigration and STN inefficiency.
Compare the STN hub with the far larger operation in Nürnberg. There, the hub works like clockwork. Certainly the lack of immigration controls for most of the hubbing international flights plays it's part; but then compare the efficiency of that operation, with proper staffing levels by AirPart (the handling agent) and the airport itself compared with the shambles this is "UK plc" (horrible expression) and UK airport handling in particular, where everything is done at minimum cost, with minimum staffing levels.
It's a shame since AB are generally acknowledged as being the best LoCo operator in UK in terms of service, and it's just unfortunate that they couldn't make the STN hub work.