Most likely one of:

  1. Noise in the detection (the room is bright etc.)
  2. The laser being a form that's not susceptible to direct modulation (if it takes a millisecond to start/stop lasing, obviously you cannot turn it on/off very fast)

High-speed laser-based systems (gigabit and beyond) don't try to turn the laser on/off at all, they just have something in front that tries to cancel out the signal (e.g., through self-interference) when you want to send a zero.