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I suspect it is to do with number of available entries (DD and GE both had 400 this year) and how and where they are advertised.
Great Escape is barely an Audax apart from in name. Ditchling Devil is known by all the London clubs these days.
The ACH rides are not marketed directly to cycling clubs as such, and entries are usually capped. I suspect that Cogidubnus Marcus JB's upcoming ride will feel like an Audax and Kingston Wheelers Rowlands RAAAmble will be like a massive club run.
Rubbish, only on Ditchling Devil and Great Escape. Straight Outta Hackney, The Shark, Greenwich Meanclimb and Steamride are all examples of where that is absolutely not the case.