As I recall, racers have worn helmets for decades but it was pretty rare for recreational skiers until 15-20 years ago. I think another factor is that helmets for a lot of kids' activities have become more or less expected, especially among demographics who do downhill skiing as a family. And, if you're telling your kid they have to wear a helmet, a lot of parents probably feel they sort of have to as well.

ADDED: I also wonder if the rise of snowboarding played a role because backward falls where it's easy to hit your head on ice is probably more common than with skis.

I don't downhill ski much these days but I did ski at Squaw Valley a couple times recently because of an event there. And I was definitely an outlier in not wearing a helmet. (In my partial defense, I do wear a hat with impact resistant material sewn in (D3O) and I'm not exactly banging down moguls through the trees these days.)