Perhaps because, if you live in Edinburgh/Manchester and want to visit central London you would take the train?

So premium traffic would only be those connecting at LHR, and if in CE/CW they get fast-track/lounge etc?

Yes, of course, there may be people with local business in LHR area - but that must be a small-ish market.

And, of course, there is the "connecting flight" issue: would long-haul travellers book JFK-LHR-EU if the last leg was not in "business"?