It's a lot more than just the existence of schedules that line up. The infrastructure has to be in place, to fast-track passengers between flights (with due allowance for inevitable delays) but just as importantly their baggage. It may be entirely possible to 'hub' through Gatwick between EDI and NAP, or London City between EDI and ZRH, and for the occasional passengers all should go smoothly, but for the massive flows at LHR (and CDG/AMS/FRA) there has to be huge investment in the systems that facilitate those flows.
And generally speaking, the transfer passengers get the benefit of all that investment for free, they tend to get better deals than the point-to-point travellers (as per recent threads on the SLF board).