The amount of points of contact on the ground make this a little bit more difficult.

When skiing, you always have at least two points on the ground for the most part. Your feet. But you also have stability rods that you can use which makes it four.

In snowboarding you are controlling one single point of contact. So your movements have to be countered by your body for stability instead of adding an extra leg.

It's the difference in balance and support.