It does though?
Even if the flight is not available on flight only, it is certainly available on package holidays. It may just be all the 'flight only' seats have been sold on these dates.
Also, the Wednesday evening flight runs 9 July - 10 Sept so more capacity than just school holidays.
The 6 flights to PMI from DSA in peak season run as follows:
Mon: 19:30
Tue: 06:25
Wed: 19:30
Thur: 06:00
Sat: 07:15 & 22:15.