[sandworm101]

Because the "beam" in a fiber is tiny and can be controlled, while the beam between two sats is actually very large and runs in free space. Most of the transmitted energy will always overshoot the receiver, resulting in significant crosstalk issues once multiple sats are in sight of each other. You cannot have two beams running the same path on the same frequency, not in space. But new fiber can be laid down infinitely along identical paths without any degradation.