Why were coax cables used for networking?
Coax was used for its controlled impedance, its bandwidth and its self-shielding properties.

Sure, twisted-pair wiring has existed for a very long time, mostly used to carry audio frequencies in telephone wiring. That isn't where the technical advancement was required. In order to compensate for twisted-pair's lossiness and impedance issues, major technological improvements in the electronics used to interface to it (such as high-speed adaptive equalizers) were required in order to make it more cost-effective than coax.