ADS-B & ADS-C - similar names, entirely different technologies.
ADS-B costs are about one tenth that for a radar site. Australia has many remote VHF radio outlets so there was little extra infrastructure required to fit ADS-B antennae & equipment.
In Australia it isn't being used to replace radar (we're updating to mode-S radars as well) but to provide surveillance coverage where none existed previously (most of the country). It's pretty "cutting edge" stuff as well so widespread adoption takes time.
You can't use it as a "drop in" replacement for radar as new aircraft equipment is required. Maybe some time in the future.... And there will always be a need for primary radar.