They explained this a LONG time ago. And said it would NEVER happen..Due to last gen not having nearly the processing and GPU power of next gen. This is just one examples Bungie gave during an interview of how this would affect the overall game. Such as, PVP wouldn't be fair, as a next gen user can see more detail at range. While last gen wouldn't even see the next gen guy, due to the lack if rendered in game pixels, and get one shot killed from a sniper.