The reason why is that powder (if ridden on at least) is more variable and bumpy with unpredictable pack density (is that a term?). Thus I have found that my board goes all over the place more and I have to constantly adjust for chaotic terrain and tire out my legs. I don't know when I'm actually going to accelerate or decellerate and sometimes even inadvertantly come to a stop....which throws off my rythm.
Ride some fresh powder that actually has some depth to it and I'm sure you will change that statement. When you get a good dump of the stuff it covers up most of the bumps and therefore makes a smooth ride. Not only that but on the steeps it helps keep your speed down which ends up being one less thing to worry about. Also on a steep groomer if you fall its pretty difficult to get back up if you start sliding as you have nothing to help you slow down, in powder you just fall into a pile of snow which doesn't hurt and gives you something to slow down with.
2011 Ride DH2 159 Wide - 2009 Burton Missions
2009 Burton Bullet 160 Wide
Riding Days: 81