I think part of it is that skiing picked up a little more "cool factor" once freestyle skiing became more prevalent.
When I was younger, snowboarding seemed like the cool young sport with tricks and baggy pants while skiing was just older people doing boring turns with lame "tricks". The X Games came along and both snowboarding and skiing became even cooler. Now youngsters probably see both sports equally as cool.
I also think ski tech has made big leaps in the last several years that have evened out some of the advantages to snowboarding (i.e. riding switch and more powder float).