Not being funny here, but what difference does it make for example if an Air India jet leaves say Delhi bound for JFK and stops off in London other than Frankfurt or CDG? Nothing!
Airlines use LHR because it serves funnily enough, London the biggest city in Europe (I think!!)
If people wanted to be shuttled over to the continent they can do so from London, like they would have to do so from FRA of CDG.
Or if they want to go to Frankfurt or Paris I'm sure they could fly direct from a major city in whatever continent they are flying from!
Doesn't Frankfurt serve more destinations than Heathrow anyway?