where there was a demand for more services from Gatwick because people didn't want the trouble and expense of travelling to Heathrow.
You're missing a fundamental point. There is of course a market from MAN-XYZ per se. However the asset used to service that market needs to be maximised, this is called sweating the assets. Hence where possible, airlines outwith p2p locos focus on particular airports where they can make most money. The potential market from LPL and LBA for long haul tends to use MAN so as not to fragment an already fragmented market. It's assured that most of this local market still uses the local airport (MAN) with some bleed to AMS. Hence the market can be persuaded to use whatever airport makes most money for the airlines, i.e. MAN. The same analogy works for Gatwick....