Its a combination of factors, but I'd guess that the current state of the economy is to blame.
Snowboarders tend to be younger. If you don't live within driving distance (8 hours or less, say) of a slope then it is BIG dollars to snowboard.
A trip with 3 days worth of riding out west would cost me a minimum of $600-$700, and that is only if I found $200 airfare,a resort with $65 lift tickets (Loveland?), and ~$50 a night lodging. I already own my own gear, so a new rider might spend another $400 more. Atlanta is a big airline hub, so it is probably cheaper to fly from here than many other places.
In contrast, If I lived close to a hill, $600-$700 would buy me a season pass to one or two of them.
Spending most of your rent on a 3 day winter vacation is less and less a viable option with disposable incomes falling in the USA.
The 20-somethings who would ordinarily replace the now-30-somethings on the hill have having very hard times finding decent jobs, which is why less of them have the cash to learn to snowboard.
Bicycles are cheap. PS3's are cheap, even booze-filled road trips to Savannah for St. Patty's or New Orleans for Mardi Gras are cheap compared to snowboarding regularly.
If/when the economy improves, numbers will go back up... unless global warming starts melting all the snow off the mountain!
That's exactly what I thought when I read the NYT article earlier. The wages for entry level jobs has been stagnate for a long time. I remember when I got my first 10/hour job(early 90's), at the time most of my friends were making 4-5/hr. I was in hog heaven, by the time that job got shipped to Mexico, I was making 18/hr. My son has been working the same place for 2 years now and just got a "raise" to 9.50/hr, and most around here still think 10/hour is a good job. I don't really see how the hell anyone gets by on 10/hr these days, things are at least 3 times as much as back in my day.
Beyond that I found this little gem from the L.A. Times, it claims older people are switching back to skiing because it's easier on the body
Snowboarding craze cools as new designs make skiing easier - latimes.com