Reckon it's all in the marketing! Which sounds more immediately appealing to the uninformed newcomer to cycling, Cyclo-Sportive or Audax?
Most won't even know what Audax is, and won't be informed by the name itself either, so Cyclo-Sportif instinctively sounds more exciting, appealing and glamourous (to most!). By virtue this alone, Cyclo-sportif has the dominant market position, the pricing reflecting to a degree the laziness of participants in discovering cheaper equally viable alternatives.