This is just a thought...

...many of the circles down this way, Cornwall/Devon are thought to have been re-erected in Victorian times. This ties in with the fashion for all things 'celtic' or Arthurian. Maybe in other places there was no reason to keep any of the old remains, and as has already been said they were done away with to make room for agriculture/industry. It is also clear that those circles that do remain tend to be on open moorland where farming has never been intensive.