Well, the price for one thing. A new paperback novel averages $7.99 and a really popular author like Stephen King gets $9.99. That's outrageous! But I don't think price is the only reason. Paperbacks just aren't as attractive as they used to be.
Check out the 50s cover above where the "cheap and evil" girl is smoked on by the giant green head. Maybe the cigarette is a burning penis. It's beautifully executed but it's also weird, and vaguely supernatural. Weird and supernatural are spices you can add to any genre, the more the better.
I'm digressing here to include a magazine cover (above) from the paperback era. In evidence are girls, guns, an exotic locale, a picture of a real, live lurid girl, and exciting letter styling. What a feast for the eyes and for the imagination! No wonder these magazines sold!
Psychotherapy (above) was a popular subject in lurid paperbacks, maybe because men envied psychiatrists. Readers must have thought, "What a job! You get to put women on a couch and listen to their sex fantasies all day long!"
Everybody likes to read about crazy people (above) for some reason, and if sex can be added, so much the better. It's sleazy but you have to admit that if this book was lying on a table, and no one was around, you'd pick it up and spot read it.
There's a genre of paperbacks (above and below) which I call "Swamp Trash" for lack of a better name. Paperbacks readers were obsessed with the idea that hillbillies didn't work and spent their whole day drinking and having sex. Could that be true? If it's not, it's still an appealing fantasy for some.
It seems to me that paperbacks of the past sold big because they gave the reader what he really wanted, even when it didn't make sense. They sold stylized hyper-reality, rather than reality. There's a certain amount of pandering in that, but the quality of the writing and artwork was exceptionally good, which means that creative people found the medium congenial, even if they didn't like to admit it. The weird subject matter lent itself to high style in the execution.
One additional reason: I imagine that more books were aimed at men in those days and men's weird and sometimes deviant tastes made for more creative opportunities in print. My guess is that most editors today are women, Vasser graduates or the equivalent, and the subjects women choose to publish just aren't interesting to a lot of men. We need more male editors.