Cost. Plus I would need 6 digital pins (3 per pedal for A,B and Z signals), which I don't have.
val=analogRead(A0);val=map(val, reading_when_not pressed, reading_when_pressed_all_the_way_down, 0, 1023);
Your original figures give you quite a bit of precision (1 in 152). Is that not enough?
int speed = map(potValue, centre+deadBand, maxTravel, minSpeed, maxSpeed)