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.