Few instructors are available and current in rarer aircraft.
Same problem for training new glider tug pilots. Since spring 2018 the system that has worked in the UK since aerotowing began whereby glider clubs' Chief Tug Pilots trained new tuggies has been ended. EASA now requires tug pilots to be taught by an aeroplane instructor who holds the Sailplane Towing Rating. The number of aeroplane instructors with the rating and appropriate experience to perform the role is minimal. The school also has to be approved or to have decared that they teach the rating which limits it even more - especially as few schools have any gliders to tow or aircraft with hooks fitted.
In practice the only places where you can learn the trade now are the handfull of gliding clubs that happen to have a tug pilot who is also an aeroplane instructor.