This may have been cover already but it seems to me they havent included trading because its what keeps you playing. In my time so far ive already done 4 exotic bounties, bought red death exotic from xur, and had several exotics come from legendary engrams. If trading were allowed i would have passed off all my extras to get items im interested in and would have little reason to keep playing. Also if trading were alowed it would shine a spotlight on just how few exotics there are, and how little content exists at this time. They need to keep you playing in order to sell you DLC in the future.