Not to forget being linked to main-stream publishers that have different editorial goals compared to era when these works were released. Not saying there were not biases back then. But now the biases are different and thus the published output is as well.

There was also the "death of the mid-list" which cut out a lot of SF books. The Thor Power Tool decision in the US had a big effect.

https://en.wikipedia.org/wiki/Thor_Power_Tool_Co._v._Commiss...

"The Thor decision caused publishers and booksellers to be much quicker to destroy stocks of poorly-selling books in order to realize a taxable loss. These books would previously have been kept in stock but written down to reflect the fact that not all of them were expected to sell."

Today, I understand mass market paperbacks are dying.