There was no movie rating system in place back in the 1940's, and movies were greatly censored by the US government. In the 1920's the Supreme Court ruled that free speech did not apply to movies, and a control board was arranged where by all film studios had to submit their scripts for censorship.
Movie studios were given guide lines by which they had to follow. So it's not so much that profanity is now widespread, as much as it was strictly forbidden back in the 1940's.
This system of governed film making slowly started to collapse over the years and gave birth to the Motion Picture film rating system. Where the film ratings and censorship is controlled by the film industry and not by a single controlling person or government.
Now that we have the rating system, films can be classified by their rating. Today's ratings are, at least by idea, implemented by our peers. So as society changes so do those who review and rate films.
That is why we see more profanity in PG rated films, as parents who rate the films accept the fact that children in this age group use these profane words or acknowledge that the film the works the way the script was written.
New ratings were introduced including PG-13. To address the need for films that target a segment of the population that are pre-adult but seek adult style films.
Still, I have to wonder who these people are that rate these films.