Luton is a low cost airport that has been updated and expanded on the cheap since the 60’s. You only have to look at the ceilings to see an example where money has been saved giving the whole Luton experience an unpleasant feel.
TUI customers are somewhat more wealthier than the average Wizz and Ryanair passenger with holidays now costing considerable sums so I would say they are somewhat more fussy. Queuing for an hour to get to the terminal was never a great start and even through those days are over people don’t forget so abandoned Luton for a better experience elsewhere. Another alternative to Luton is Birmingham where TUI have 11 aircraft based.