The people who listened to those artists are dying off and there are far fewer people alive now who know who they are. Only the real legends hold value or see an increase. Sinatra, Crosby, or someone who was a big enough name who also happened to die young and so the supply if limited. Buddy Holly, Patsy Cline etc. Doowop was a short-term genre and there were a LOT of people doing it, and I will bet if you read a list of some of the bigger names to a 30-year old, they wouldn't know more than one or two, and to a 20 year old, you'd get a blank stare. Motown? Some of the bigger names are still living. Diana Ross, Stevie Wonder and the like. If someone is still alive to sign, that depresses the market.