why old 50s-60s autographs have little value?

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.

Commodities have high value when the supply is low, but the demand is high. Apparently, that's not the case with old 50s-60s autographs.

To have value something must be rare, there were thousands of signed photos sent out every week.

With signed photos, as with antiques, there's no value except in the rare instances of a shop located where people of high interest live. For example, you will find a high interest among Hollywood and Beverly Hills tourists in celebrities such as the great singers of yesteryear. Therefore, your autographs have great value, but only in specific locations.

Because the people that listened to the artist you mentioned are dying or dead.

Well go on then, tell us why.