Why coaxial cable not used in low frequency?

Coax is normally used in high frequency applications due to its ability to shield the signal. The coax construction prevents radiation, while at the same time prevents noise pick up. It is the shielding that makes coax a better choice at higher frequency, however coax can be used as low as audio frequencies.

Lower frequency systems do not need the shielding due to the long rise times. They are not as sensitive to noise pick up and generally do not radiate.