I agree that cost is a big impediment - always tends to be. But, while the cost of a lift ticket or a season pass is not cheap, I don't think it's the cost of skiing itself that is the issue. Rather, I think it is the "all-in" cost that can become an issue for people, in particular for millennials who tend to have less discretionary income.
With season passes like the Epic Pass affording a break-even point after just 6-9 days of skiing (depends on resort, peak/low season, adult /child), skiing itself is not all that expensive. Even if you purchase day lift tickets for $80-$100, that is still not a terribly expensive proposition, considering how much a four hour round of golf at a decent course often costs (around $100 where I live in the NYC area) or how much it costs to rent a tennis court for just a couple of hours (around $100 where I live in the NYC area).
What I think happens is that all the other costs begin to add-up.The majority of skiers are not driving 1-2 hours to a mountain (most people will drive under an hour to play golf and 5-15 minutes to play tennis). For a lot people, skiing requires spending a fair amount of time driving and that means money on gas, tolls, parking...and in some cases may require an overnight stay. Lodging at ski resorts is not cheap, at least not near the mountain, and it can equate to as much, if not more, than the cost of skiing itself. And, while many skiers bring their lunch, it is not always the case since carrying your lunch around as you ski is not that easy or attractive for many people. Food at ski resorts is not cheap either (nothing is), so you can easily spend a fair amount on food.
Then you have equipment costs. All you need for tennis is a $100 racket, some tennis balls and sneakers. For golf a $500 set of clubs will often do the trick. Skiing equipment can get very expensive once you add up the cost of skis, bindings, poles, pants, boots, helmet, gloves..., And, skis need to be waxed/maintained and a lot of ski equipment tends to have a fairly short life span (3-5-10 years max).
But studies show that cost is not as much of a factor as weather / conditions. And we all know what has happened - increased weather volatility. While the average person may be open to spend gin money to go skiing, if they have a bad experience, they will likely not go back, at least not for some time. When it comes to skiing, weather matters more than just about in any other sport (no one likes to play golf or ride a bike in the rain but people do - you can play golf or ride a bike in a huge temp/weather variance). And while snow making can often mitigate the impact of lack of snow, it can not offset temperatures above freezing.
Millennials happen to be a demo group that also has more options today than folks there age had in the past as it relates to how to fill their free time. Travel is cheaper and more efficient, technology has made entertainment such as video games and movies more engrossing and accessible, as well as changed the way millennials socialize (chat on Facebook vs getting together in person). Even as it relates to sports, millennials have gravitated to other sports like triathlon, mountain biking, adventure racing, mixed martial arts, yoga, cross fit,...all of which were not very popular 30, 20 or even 10 years ago.
Below is graph that shows annual skier visits across the four major US ski areas over the past 20 years. On average, the results are flat at best (some growth in the Rockies, flat in the Midwest and down in the East and the Pacific).