a small regional town with a relatively small population base (approx 40000) and a major city like London
Saarbrücken (200,000) is 40km to the west of Zweibrücken and no doubt Ryanair were actively targeting this population who have no direct route to London from their local airport. Who knows the exact reasons why the route is being cancelled. Low demand / yield or a hissy fit between Ryanair and the airport when the introductory reduced fees turned out to be just that:
introductory. Ryanair have been known to cut profitable routes just because they weren't getting their way on charges with an airport. So we'll never know I suppose.
Winners: London routes to Luxembourg, Hahn, Frankfurt, Karlsruhe, Strasbourg.
Losers: People wishing to fly between London and Zweibrücken / Saarland / North Eastern Alsace.