travelling to the USA - why would I take a short flight to DUB, change gauge for the second sector when I can use my local airport MAN and fly a single sector to get where I want.
That would make restrict you to JFK, EWR, MCO, ATL, LAS, PHL and the Thomas Cook program. Quite a lot of people connect at the US hubs, so it's often a one stop journey. Now if you fly MAN-LHR-xyz direct, you are supporting more UK based jobs and investment, now I am not a protectionist by any means, but it does offer the likes of LBA a huge volume of one stop options via London to fly from their own local airport. This then is the trickledoen effect in action.
You think INV is not looking to get a connection to LHR? They know the inward investment that's going to bring on a one stop single ticket (oneworld, STAR or Skyteam) to the Highlands from a huge number of world cities, a market easyJet quite rightly, don't do.