One thing that people rarely discuss about book publishing is a change to US tax law in the late 70s that meant that publishers couldn't write down the value of unsold inventory, but could write off that inventory by destroying it.

That meant that poorly selling books were destroyed to realize a taxable loss, which killed the ability for books to slowly "pick up steam" over a year or two to eventually generate a profit for the publisher. If you didn't make a profit fast, the backlog got destroyed and the book lost its chance to make money.