I think the reason Hollywood usually showed boxers in previous days was modesty. They were reluctant to show the bulges which are normal in briefs. Briefs have never gone out of style, altho boxer briefs have taken some away. Some guys like the pressure of a brief on their cock. Others with draggy balls find that boxers don't give them the support, and boxer briefs sometimes let a ball slip out.
I don't agree that "briefs have never gone out of style." they were once the choice of, say, 9 out of 10 guys. Now boxers and boxer-briefs have a majority share of the market.