I know this is a dead beat topic and I still don't know why Bungie didn't respond to this request. To be honest it died a little bit after a while, probably since a lot of people just quit Destiny. The lack of local co-op became painfully apparent when a buddy of mine was at my place. It would be fun to play together, but alas he played story mode and lost interest the moment random team members abandoned his party.
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Likely to push more sales. If you have a friend you'd really like to play the game with. With Split screen you'd just invite your friend over and he'd play for free. Without, he has to buy the game. There may be honest, technical reasons. But I wouldn't be surprised if profits was one of the big reasons.