Question

why medium of core in optical fiber is not air?

Answer 1

it'll have to be a light dense medium(relative to the cladding) for total internal reflection in the fibers to ocur.

Answer 2

Light should travel in a medium of higher refractive index than the cladding material,. otherwise there won't be Total Internal Reflection.
Air has the least refractive index after vacuum

Answer 3

i think you could have asked why we need cladding in the first place.. why won't just a glass tube (core) do the job?