Night of the Living Dead became a slow-burning sensation, making back its budget a couple hundred times over at drive-ins, grindhouses, and matinee showings. Romero was credited with not only delivering an unexpected hit, but a searing social commentary that redefined an entire genre. Night of the Living Dead was a success, in every possible sense of the word.
https://daily.jstor.org/the-d-i-y-origins-of-night-of-the-living-dead/