Ged my theory on that is it's because it was played post 77. When the returnees returned en masse, in the mid 90s - as you say they wanted to hear familiar records.
In the main this was records played during the "golden period" of say 72-76. Hence they never heard Betty Boo, (and Cecil Washington, Vicki Baines and all the big sounds of the latter casino years) when they were in their prime.
Your other points are spot on, loads of good records fall into that space between the oldies and what the big time DJ's play