Since no one really knows for sure, and the fact that I do not like anyone's answer so far, I will chime in....

I would say that the reason would be that btc-e offers more privacy protections then bitstamp, therefore people are more inclined to sell on btc-e then they are to buy on btc-e as they do not need to go through the AML/KYC verifications that bitstamp (and other exchanges) require their customers to go through