Because our NHL defense is full of sh!t like Salvador, Harold, Volchenkov, Merrill and Gelinas (with their rookie mistakes). And, Lars was playing well up here. Furthermore, the NHL club has not been playing well. Thats why he has reason to be disgruntled. Fortunately, he doesnt seem to have that attitude.
By that reasoning, every young player in the AHL should be disgruntled. Larsson started playing well when he got the added responsibility of playing with Gelinas. Then he missed 21 games with an injury, but he was starting to show he could take the next step. Where is a good place for Larsson to get used to playing big minutes, on the powerplay, on the PK, and in important game situations? The AHL. It will do wonders for his development to get used to the added pressure. Larsson wasn't going to decide the playoffs. They want him able to play 24 minutes like Greene does.
It's the same idea as having an assistant coach in the NHL, then moving him to head coach in the AHL and seeing how he does in the position before making him an NHL head coach.