The thing is as we are all aware Luton isn't London, Huge enough catchment area that takes in so many random surrounding cities/towns with decent sized populations that are miles away from Heathrow, Stansted & Gatwick.
Luton, Bedford, Milton Keynes, Ayelsbury, Bletchley, Oxford, Stevenage, Northampton etc etc St Albans & Watford are closer to the "London" Airports but still enough pull to compete on routes. In terms of distance isn't comparing Luton and Gatwick (74 miles) along the same lines as comparing Bristol with Bournemouth (77 miles), Not close at all.