Why do optical signals used in fiber optic cables have a very short wavelength?

The pulses of light used in optical fiber are in the infrared or visible spectrum, which is shorter wavelength than the microwave radio spectrum. Neither microwave radio wavelengths nor ultraviolet spectrum wavelengths can pass through the materials used for fiber optic cables.

In the early 1950s A. T. & T. had begun laying a system of a few thousand miles of underground microwave waveguides (similar in purpose to modern underground fiber optics systems) but the system was never finished and ultimately proved to be not cost effective. But the cost per foot of fiber optic cables is nearly negligible compared to the cost of metallic waveguides, the diameter of fiber optic cables is much smaller compared to the diameter of waveguides allowing many more cables in the same space as one waveguide, and the shorter wavelength allows wider bandwidth signals on each of these fibers than the waveguide could carry.

Perhaps someday fibers might be able to be made of material that can allow ultraviolet spectrum wavelengths to pass and even wider bandwidth signals could be sent, but not now.