Optical carriers are in the region of Hz to Hz, i.e. three to six orders of magnitude higher than microwave frequencies. However, the atmosphere is a poor transmission medium for light waves. Optical communication only became a widespread option with the development of low-loss dielectric waveguide. Optical fibres have very small diameters. A very large number of fibres can be carried in a cable the thickness of a coaxial cable.
Optical fibres are almost completely immune from external fields.
They do not suffer from cross-talk, radio interference, etc. It is difficult to tap into an optical line. It is extremely difficult to tap into an optical line unnoticed. Low transmission loss. Modern optical fibre now has better loss characteristics than coaxial cable.