The simple answer is because Istanbul is their main hub and it makes operational, and economic sense for them NOT to fly directly between the UK and Dalaman.
Like all transport undertakings, they employ traffic and economic analysts, and if they thought it was sensible to fly direct, I'm sure they would have persuaded the powers that be by now.
Just because an airline gives up their slots for take-off and landing they don't come free to another airline. I've no idea about current prices but it was reported in 2013 that Delta Airlines paid $47million for 2 pairs of slots at Heathrow for an unspecified period.
With the tourist industry in Turkey nosediving, it would be economic suicide for an airline to start buying extra slots to what is, to them, just a bus stop airport.