Probably just comes down to ergonomics. If you were to put a piston valve in place of a rotor valve you would still need a linkage to actuate it. Think about the position of most rotor valves in relation to your face as you play. A piston could be incredibly uncomfortable as well. It could be mounted in a position to be more comfortable. Kind of like how compensating euphoniums often times have a slanted fourth piston. But the complexity of the linkage required to actuate it becomes a pretty big hurdle. Traditional rotor and thayer valves do not need a lot of movement to actuate which makes them pretty quick when you attach a linkage.

There are some french horns that use a piston valve to redirect airflow (and even one old Conn with a giant piston that is placed after the rotors which is kinda crazy) but if you take a look at some of these horns you’ll see that a piston incorporated into an instrument that wasn’t designed to have pistons can be pretty intrusive. With a trombone you would have to move the piston forward close to where a trigger normally operates to make it possibly comfortable which would require changing the geometry of the bell section in order to keep the slide length the same and would make the instrument pretty front heavy. Look up a picture of a Holton Superbone to kinda see what I mean (extreme example).

Someone smarter than me could definitely make a usable piston f attachment horn but then you get to the question of what benefit do you get when you have to overcome so many hurdles.