Why do we use optical fiber instead of coaxial cable?

simple

because it is more secure, offers more BW for considerabley long distance vs co axial and is not sensitive to water and chemicals etc

and also it is harmless as all it carries is light :)

Without losing power, fiber optic cables can carry television signals for a very long distance because of the technology of using very thin strands of glass. In different areas of the network, both single mode and multimode signals will be used. Signals will be sent from the central office to optical nodes using single mode fiber and then it will be converted to multimode.

Optical cables use light signals instead of electricity and this makes the signals much clearer and offers a very low signal loss over a wide range frequency with no interference to other signals.

Within the next few years, most cable television companies will have fiber optics installation, making it an advanced and versatile communications network solution.

Because opical fiber faseter & resistance free than coaxil cable

More capacity and Less atennuation . Pereferable for longer runs

Faster , Reliable and more secure

Because the optical fiber cables have much activity more than coaxil cables in transmision the signals 

Greater capacity, cheaper production