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