Year to year fluctuations are mostly about snow. It's not an accident that the record high skier visit seasons were 2007-08 and 2010-11.
It has been my observation over the years that if it is a poor start to the winter with so so conditions at Xmas, then they decide it is a poor snow year and then tell all their friends when they get back home and they reduce or eliminate subsequent ski trips. It almost doesn't matter what the weather in the mountains is like for Jan. and Feb. The reverse is true if there are piles of snow everywhere and frequent snowfalls during the Xmas holidays.
There is some truth to this and it's a big part of the reason 2001-12 is the low attendance outlier of recent seasons rather than the worse overall snow year of 2014-15. Mammoth's season attendance correlates slightly higher to Nov+Dec snowfall than it does to snowfall for the entire season. Rusty Gregory at Mammoth blamed depressed attendance in 2011-12 at Mammoth on the widespread negative publicity about that season in the national media. And of course for most of the national media the sun rises in New England and sets in Colorado as far as skiing is concerned. No matter to them that skiing was quite good at Mammoth from late January onwards or in the PNW and western Canada for the entire 2011-12 season.
Ski areas need to analyzed individually for drivers of attendance. Destination areas are less sensitive to year-to-year snow fluctuations and attendance is based upon the broad range of factors that affect advance purchase decisions. I have to believe that advance purchase visitation to Tahoe is being eroded by the cumulative snow record of the past 4 seasons. Mammoth's market is 95% drive-up, so yes it may drop more during the bad seasons, but I expect it will also recover promptly when the snow is good, as it did with the strong early season in 2012-13.
Other ski resorts, regardless of their average annual snowfalls, would be wise to follow Sun Peaks' summer grooming lead.
That's a very limited option at steep and rocky mountains like Squaw, Taos, Crested Butte and Snowbird.
The rest of this thread has been about the long term growth of skier visits and covers the same ground as numerous other threads. You need to look at the long term trend over decades to get rid of the short term noise created by good/bad snow years. The trend of the past 15 years was mildly increasing as the Millennials entered their prime skier demographic years. That trend would have shown a steeper increase if not for the cost/alternative recreation factors cited in this thread and elsewhere. The next generation is somewhat smaller so the immediate future trend is likely to be flat, with US skier visits in the 57-58 million range in average snow years. Increased international visits might bump that up a little bit.