So, they have this glass in the middle of the fiber optic and a small layer of a different glass on the outside. The outside glass has a higher refractive index than the glass inside, which makes the total internal reflection possible. There's problems with bending the fibers which can make them less efficient, not work, or make their efficiency deteriorate even if you bend it back.
A huge problem is with making a clear signal over a long distance. Over time, an exteremely short light pulse can spread out over time and requires expensive repeaters to go over long distances.