The length required is for specific aircraft and depends on the altitude, temperature and weight of the aircraft, not to mention runway conditions (braking action, etc.) Airports build them large enough to accomodate the aircraft which need to land there. Some airports have shorter runways and cannot accomodate larger aircraft - LGA, for example. Clearly lack of space is a factor which inhibits expansion of runways to accomodate larger aircraft or more frequent flights.