Why has everyone forgotten the problems when Blackboard Jungle was first in the cinema?
The film came out almost 70 years ago - Thousands of films have come out since then, and it's largely been forgotten.share
The majority of the cast is now deceased - The film's cast has largely fled this mortal coil, and so there aren't many people around who can speak to its impact on them.
The film wasn't particularly successful during its run - The film didn't set any box office records when it aired and so it was never more than a minor “blip" on the Hollywood radar.
The film is topical. Painfully so - Basically if you lack knowledge of the United States in the 1950s, the film can be nearly incomprehensible. It was made for its time, not for future audiences.
It's not very good- Glenn Ford was decent, as was Vic Morrow; and Sidney Poitier does his usual workmanlike best. However the rest of the cast mostly “phone in" their roles. The writing is also trite and the direction is spare. It's not a very good film beyond Glenn and Poitier's performances, and its being forgotten is the result.
Basically a bad exploitation film from nearly 70 years caused minor problems at a few venues, and a sensationalist media decided to hype that up. Since their doing so didn't hurt ticket sales, the studio didn't bother to protest the media coverage and made a few dollars as a result.
That would be barely memorable now with the Internet and social media. It's forgettable after nearly 70 years and regarding a minor box office film hit.