It's a feedback loop - smoking was advertised as being cool way back when, which led to movie characters smoking to appear cool, which then reinforces the advertising.

When there was a push to regulate smoking in advertising, it cut the original feedback loop which made film/tv characters not use smoking as a sign of being cool. This led to advertising (if it were allowed) to be less effective at portrayal of coolness via smoking.

It's not a simple one-to-one cause and effect.

It was cool to smoke when all the cool guys in the movies were smoking. One of the reasons it’s not cool anymore is that the cool guys in the movies don’t smoke nowadays (although they do it more often now than they did ten years ago; which is worrying).

The correlation between an exposure and initiating smoking is proved: https://pubmed.ncbi.nlm.nih.gov/27043456/

It’s not that people stopped but that they didn’t start. Smoking was no longer sold as cool so kids didn’t learn of it as a cool thing.

Kids and even adults pick up cues from games, movies, books. War is like CoD and heroic war movies (why do many 18 year olds go to the army expecting glory and come back with trauma and broken dreams?), sex is like in porn, and gangs are like in GTA. Until they gain practical experience and slowly realize some things are vastly different. Maybe a couple will love “porn sex”. Most others will break a leg having shower sex and reconsider the “teachings”.

Characters in movies stopped (or reduced a lot) smoking pretty much across the board.