In France it has taken time, and there are still some holdouts (most notably the national ski school, ESF).

I believe it's mandatory already in several Swiss resorts, I know it is in Zermatt.

I really hope the ESF will mandate helmets for all instructors, this would go a long way to normalising them.

I don't think wearing a helmet when skiing is comparable to commute cycling. I think motorcycling would be closer. In both cases you're going at relatively high speeds with quite long breaking distances.