There used to be a Chiang Mai based airline that operated a grand total of 2 aircraft both Cessna Caravans each taking something in the region of 8 passengers. Name was Kan Air and they flew to places like Mae Sot, Pai, Chiang Rai, Nan and Mae Hong Son. Name was Kan Air and an absolute joy to fly with them but alas long gone. The aircraft were for a long time parked on the military side of the airport but not sure if still there.
As stated not a lot of demand from Chiang Mai to Mae Sot and elsewhere considering there are a grand total of 2-3 flights from Bangkok. Maybe if the border was to reopen flights would pick up but not sure that will happen anytime soon but heres hoping.