Actually, it's a good question! Athleticism goes a long ways toward making up for "uncouth" technique in the bumps, so many young, athletic, aggressive skiers are more than competent in moguls, even without much technical proficiency.
But powder, sweet and effortless as it is to the expert, requires a bit more sophisticated, consistent, and polished technique. Defensive, braking movements work just fine in bumps, as long as your body can take the pounding that high-speed hammering causes. But braking is nearly futile in powder. In powder, and especially heavier or broken, inconsistent powder, it is nearly essential that the skis move the direction they're pointed, so that they slice through it cleanly. Getting them moving sideways takes a great deal of effort in the first place, at the very least, and skis moving sideways in soft conditions are just looking for trouble!