Greater variety of airlines and routes/frequency's (Less so of the latter for LTN) and neither airport has anywhere near as much dominance by one airline as STN does with Ryanair. Better rail links and also locations (Luton is good for a lot of London + Oxford/Buckinghamshire & Gatwick has South London + Kent, Surrey/Sussex, Hampshire) also play a part along with onward connectivity mainly @ Gatwick. Stansted still has a strong catchment and high demand but these factors are the main reasons that many people choose LGW or LTN over STN.