The economy, increasing prices of lift tickets ($119(!) at Vail), lack of decent snow for the past few seasons (Colorado is what, at less than 60% average snow pack for the season currently). I think this all contributes.
Judging by my nephews and their friends, I think snowboarders tend more toward the budget side of things then their skiing peers too. They'd rather build a ramp in their backyard or do rails at school yard then pay for a ticket to do similar stuff at a resort.
MSN Money had a very similar article
Actually, I think Vail is $129 a day now.
As for budget of snowboarders vs. skiers - I think its much cheaper to snowboard. Top of the line snowboard boots cost what, $400 maybe $500? My skier buddy (actually, most of my friends are skiers here) just dropped $1700 on new boots. $1700. That's more than my rent in the Denver suburbs combined with my seasonal rent in the mountains. High end snowboards cost $600-700, almost all skis cost that much. Bindings are super expensive for skiers as well. Shit is crazy, actually.
That all said, I actually enjoy riding with skiers. Ever been in a flat and think... damn I'm gonna have to unstrap and start skating? With my skier buddies around, they'll be ahead of me and use their pole to slingshot me forward. Not that that's the only reason, but its definitely a plus.