One of the main reasons, at least for take off, is that Ryanair's 737-800 aircraft engines are de-rated so to save fuel, engine wear and maintenance costs. So with more fuel needed for longer routes / higher passenger loeads needed, the aircraft would take a longer stretch of the runway for takeoff with the de-rated engines. It wouldn't be cost effective to up-rate the engines just for the few destinations FR could potentially serve from airports such as BHD with shorter runways.