Good question. I've had a bear of a time sourcing good info on this and would welcome a solid analysis.
https://en.wikipedia.org/wiki/Line-of-sight_propagation#Radi...
There could also be timing related constraints that limit max range: radio waves propagate at the speed of light which is fast, but not instant.
The issue you have with long distances is multipath, which can throw off the complex timing required to have multiple users in realtime on a single radio.
I can't find a direct reference to it on wikipedia, but I suspect this is what the Vortex satellites were doing: https://en.wikipedia.org/wiki/Vortex_(satellite)