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Yeah, I think you're mistaken...Horgmo and McMorris both did triples on the big air jump.
The problem is STP builds a long, flat jump step-over with very little pop for their "big air" jumps, where as for JOI it's a shorter (but still big) true table with a ton of pop. Snowboarders can naturally get more pop (which is why a lot of them really enjoy urban rails with a flat takeoff, i.e. no lip), so they're able to get triples around on those kinds of jumps (plus they're triples are basically 3 backflips and a 180).
It's also interesting to notice how the skiers preferred the slopestyle jumps to the big air jumps (those jumps had more pop...the 3rd jumps was similar to the JOI jump, just more cut out), while the snowboards had a terrible time with the slopestyle jumps (eliminations was basically decided by whoever could make it down the whole course), but were able to throw down a bit better on the big air jump.