at the risk of sounding like the North/South divide, the reality is that most people traffic is from the South, so that's where the demand is and where the majority of flights have to originate from. i'm from the South West, it may take me 2 hours to Heathrow by Coach (train is quicker ) and 3 hours to Gatwick, but to be honest I would rather do that than fly from my regional airport which is 30 mins away and lacking the facilities of London. For early flights I book overnight near the airport I'm going from, am happy to do that and treat it as part of the holiday.
i really cant understand why people FLY from Manchester, its a few hours from London by car or train! Booking in advance gets a cheap rail or coach ticket, so it must be more expensive to fly, and I cant see that its any easier than going to Heathrow or Gatwick (I have flown from Manchester, so have experienced the Airport).
London is more Centric based for most people and easy to get to, if it was reversed and most flights were from Northan Airports, I think a large porportion of customers would not go and use another holiday firm. At the end of the day its down to demand and cost, and flying from London Airports for most people and tour operators is the cheapest and eeasiest option.