Switching. The faster you can switch something, the more data you can carry over the channel. Because LEDs work by exciting a material, that material has to switch between energy states. Lasers, on the other hand, use the energy states of photons (If I understand it correctly - it's been a while). So basically the inertia you have in the physics of an LED, while orders of magnitude lower than the inertia of a filament light source, is still orders of magnitude higher than a laser.