In short-range, low-data-rate communications, LEDs are often utilised. As far as I can tell, the biggest issue with using LEDs for long-distance communications is that they are difficult to couple with optic fibres.

Single-mode fibres are utilised for long-distance communications, having a small core (usually 10 microns in diameter) and

It accepts light from a limited number of angles (typically a cone of about 22 degrees around the straight-ahead position).

Lasers can easily produce collimated beams with a well-matched cone angle that can be focused to a spot smaller than the core size. A laser transmitter that draws 1W and emits 100W is inexpensive and simple to construct.

If you take a lens and focus an LED, you get an image of the LED chip - it’s really quite difficult to make that image <10um across, but even if you do (with e.g. an expensive high-NA objective lens), the cone angle of the focussed light is very steep, so only a tiny fraction of the light is actually coupled into the fibre. You are running into the fundamental limits imposed by the Etendue of the system.

Since you can’t get much light into the fibre from an LED, this limits how far you can transmit without a repeater.