The early movie producers were raised on great literature, and made movies of great books.
Movie audiences of the 40s were assumed to be somewhat familiar with classic literature and there seemed to be a demand for it. These things are still being made—but usually for BBC/PBS and not the big screen.
It might be because of the advent of TV in the 50s. Except for movies, people either read books and articles, or listened to stories/plays on the radio. The scenes unfolded in one’s mind. A well-made movie brought the imagined scenes and characters to life.
‘Great literature’ is not all that popular a read anymore and is actually being frowned upon on college campuses, so fewer are being exposed to it. Comics or perhaps comic-based movies are what is popular now. Sigh.
Even when populaar, it was still superior literature: Gone with the Wind, Rebecca.