Yes, excluding the commercial terms between the airline and the airport (although I guess we can assume that TUI & Thomas Cook will be able to get better deals than most other tour operators/airlines), I thought the issue was the volume of passengers on each flight.
The airport is currently geared around flights of typically 60 people a time flying to Amsterdam (and less to Aberdeen). The passenger traffic is also spread out throughout the day focused on business people who dont bring huge amounts of luggage and are not so price sensitive.
A TUI 738 takes 189 pax and is a far bigger larger logistical challenge as it causes a one time spike in demand for the airport's services (check in, security, baggage handling, catering etc). A couple of flights a week from an airline paying little-or-nothing in landing fees probably causes more trouble than it is worth.
Unfortunately, unless MME can get an airline to commit to a decent schedule or a based aircraft, small scale charters and business routes are probably the only route to go down.