To do with the transition from hunter-gatherer to agriculture and settled society. As you saw they were symbolic sites of meeting and huge cultural power - you don’t necessarily want that in your own backyard but somewhere neutral and where everyone can access.

Having said that the climate was more temperate in that region 10,000 years ago (during the Mesolithic) and subsequent users could have been driven or moved out and it no longer had relevance in their lives. No culture lasts forever… but their monuments might.