Here's the reason for the inflection point (circa 1975). Several publishers decided to do the unthinkable - give best seller treatment (hardbacks, displays, advertising, promotional tours, ect.) to top genre writers.
Before then, the purpose of genre writers was to be the bottom feeders of the publishing world. Good for profit, but not worthy of other notice. (after all, they weren't literature.)
The first experiment was Herbert's Children of Dune (1976). To the great surprise (and horror), it sold massively. Thereafter, top genre authors started getting the "best seller" treatment for their new books. And they sold (even if they weren't always so good...)
And the rest was history...