Other thing to bear in mind is the airport may geographically be only 20mi away - but the the drive time could be considerably more.
For the sake of argument, take Biggin Hill. Quite a few city types live fairly local to EGKB (within 30-45min). But it's a 90min drive to Farnborough or even worse to Luton. At the light jet end of the market your average punter may well say "pfft, i'll drive" - but once you start getting into the bigger stuff - midsize and upwards - one can afford to have one's jet come and meet them.
For aircraft owners, hangarage can play a big part as well. I know of at least 5 aircraft based at Farnborough/Stansted where there is guaranteed hangarage available, yet they live in (or have business in) North London and regularly fly from Luton (where hangarage isn't guaranteed).