Why can helicopters not fly as fast as airplanes?

In order to fly, aircraft need to produce lift. In airplanes, lift is generated by the air that's flowing over the wings. The faster the airplane flies, the more lift is generated. In helicopters on the other hand, lift is generated by the spinning rotor blades. If the helicopter is in motion, the blades will produce more lift on the side where they are moving in the helicopter's direction of travel (advancing), than on the side where they are moving in the opposite direction (retreating). This is called dissymetry of lift. The faster the helicopter flies, the bigger this dissymetry will get. The helicopter's maximum speed is equal to the speed at which the retreating blade would begin to stall.