The main thing to blame for that was WWII, where a lot of people learned to read. When they returned, there was a deluge of inexpensive pulp novels and magazines, in addition to abridged and simplified versions of classic or important works (notably from Reader's Digest.)

Many of the adults consuming these books had just learned to read. I adore that stuff and collect it (for both lurid cover art and lurid content), but the actual gems are not so common and 98% of them are completely unremembered.