Though I do very little downhill skiing these days, they're extremely common at US ski areas now whereas they used to be almost unheard of except for racers. (I don't wear one but I do wear a hat that has ribs of deformable material.)

I don't remember a big campaign or anything but probably some combination of snowboarders normalizing, it becoming seen as negligent not to make kids wear them, and probably a general increase in safety culture--especially among the sort of people who can afford to downhill ski.

There are only around 40 downhill skiing deaths in US annually--although the majority of those are brain injuries.

The landslide was 2009/2010, after the Ministerpräsident of Thuringia caused a really bad accident, which he survived but the woman he crashed into didn't. He was wearing a helmet, she wasn't.