According to David McDonagh from EA Sports: "Skiing was actually a feature that was a part of our initial design plans. But it was something that we had to cut awhile back given our production timelines for the game."
However, the reason it was cut was that there simply wasn't enough man-hours to deal with the complexity of skiing and snowboarding. McDonagh revealed that they would need "literally twice the number of carving, grinding and trick animations in the game" , but the main reason is because SSX is a snowboarding game and not a skiing game.
"splitting our focus here would result in far less unique trick content, which is really important to our fans." said McDonagh.
EA Sports, of course, has not ruled out including skiing as DLC - but it wouldn't be an automatic inclusion, either. McDonagh stated: "decisions for what our final DLC plans look like won’t happen for awhile." EA is adopting a wait-and-see attitude towards skiing in the game.
SSX will be developed for the PlayStation 3 and Xbox 360, but no release date has been given yet.