I feel like a lot of it is that we, women skiers, are just not to that level yet. I love to ride park, but still struggle with urban on. I've gone on some urban expeditions with the boys, but the rails they're hitting, quad kink, closeout to 10ft drop, etc., there is no way I could hit those or the other girls I know. So I help build the run in, landing, pull the bunjee and film. Most of the time urban features are more complex, more kinks and turns and drops that, for me at least, are just way above my skill level. And I feel like for the average girl skier it's the same way, if I could fin a rail/wall like the one EricaMN is hitting I so would, but my area doesn't really have "easy" urban.
Second, you really need a group of people to make urban happen. The boys don't want to take the time to set up an easy urban rail when they can go hit a quad kink. It's just how it is, and me and the only other freestyle girl I know aren't going to be able to set up everything needed in a nice amount of time just for a easy urban rail.
And as for the snowboarding girl dilemma. A lot of it, I think, is that they are still a little more advanced in progression. They also have a bigger group to pool from. Snowboarding is still more popular among girls than freestyle skiing. Therefore, you can have more girls do more risky things.
In my personal opinion its way easier to hit an urban rail on a snowboard than skis.
It's not about the balls to hit an urban rail, its about the ability.
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