For me it's a practicality issue. The places I tend to visit on UK trips are mostly located along the Thames Valley. A hotel near Heathrow is a convenient base. Gatwick is really out of the way. A 90-minute bus ride (or drive) after getting off a seven-hour flight is not my idea of fun. If I have to be in London, I have no problem flying into LGW, and have done so several times. An extra 15 minutes on the train is no big deal.