It's been over a decade since I last stepped foot in a ski area. I don't remember anyone wearing a helmet that wasn't on a snowboard. Are they more common now? It makes a lot of sense with how bad things go when they go bad plus all the trees.
It's almost ten years for me and the last thing I remember was a landslide shift from "hardly anybody" to "almost everybody" within only a few seasons (Austrian alps).
Never read any triumphant reports about how this has saved ten thousands of lives though, so I suspect that numbers aren't that impressive.
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.
