Gatwick is recognised as a proper London Airport while many still laugh at Luton and Stansted being London Airports even though from memory Gatwick is 28 miles from London while Luton is 32 miles and Stansted is I think 33 miles or is it 35 miles?
Not much in it but Gatwick is the first choice for many passengers as it is more premium than Stansted and Luton and for historic reasons for no other reason than they have always used it. Luton still has an image problem that it has had for 50 years and still can't shake. Stansted is Ryanair, which puts some people off. Some of the other airlines there will be temped to move south of the Thames.