Why do line of sight communication links have a restricted range for a given tower height? Is it only because of the Earth's Curvature?
As pointed to by Marcus' comment, "line of sight" communication is just that: if you can see it, you can communicate.
There are formulae that will calculate the distance to the horizon, given the height of the tower at each end, and it is because of the curvature of the Earth, as you suggested.
In addition to earth's curvature, obstacles in the various Fresnel Zones may reduce received signal strength, and hence lower effective range