If I remember right, a propeller blade makes lift (thrust) just the same way a wing does. Lift is proportional to the square of velocity. If you slow down propeller RPM for takeoff, the velocity isn't there, and neither is the thrust. And trying to run a high-pitch prop at high RPM at low airspeed will just cause stalling due to excessive angle of attack.
A fixed-low pitch prop would have to run at excessive RPM to generate a positive angle of attack at high speed. Even though the motor might be able to handle the RPM, the propeller tips would go supersonic, or perhaps the prop would just fly apart from the centrifugal force.