Question

why carbon is used as a reducing agent

Answer 1

The energetics of the following generic process :

2MO+C⇌CO2+2M2MO+C⇌CO2+2M

are favourable enough for the equilibrium to lie mostly to the right. Carbon dioxide being a gas which does not react readily with the mostly liquid metal, escapes easily.

One important consideration in large scale metallurgical processes like extracting Iron is cost, carbon in the form of coke, which in turn comes from coal is a pretty cheap reducing agent. That’s why steel and iron works are either situated close to coal mines or connected with them for assured supply.

Answer 2

Because it is a non-metal and it combines with oxygen and forms its gaseous non-metallic oxides.