Never read any triumphant reports about how this has saved ten thousands of lives though, so I suspect that numbers aren't that impressive.

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.