Must be the construction; it takes a lot to make a burly pow ski. First of all, consider that many people that ride pow are bigger than their park-riding siblings, so you are looking at bigger skis already (180cm+) that need to be thick (more weight), fat (more weight), and likely use different technology focusing on the strength of the ski rather than the lightness of the ski.
I'm assuming, of course, you're asking about bandits, Maidens, ARV 185s, etc.