I beleive it was the complication factor, and the expense...

MK marketed them in the early 80's and in fact, Hanno Prettner won the 1980 world champs with a magic fitted with an MK constant speed prop...

the complication was having a 60 sized aeroplane with all this gear jammed in it...

1.ailerons
2.elevators
3.rudder
4. throttle
5. gear
6. flaps (snap flaps or speed brakes)
7. in flight mix
8. prop pitch.

the fuselages were crammed with stuff and weighing in at 9 and 10 pounds, these babies were guided missiles...