I don't question that, but why do people pay more? Two factors - 1) quality of terminals (although only really relevant for T5) and 2) transfer time to central London could be addressed with new facilities and a big surface access upgrade at LGW.
Terminal facilities at LGW North and a rail link in place faster than LHR's into London were superior to anything at LHR until recent years. The market prefers LHR.