I think it is to do with the scale of the new playable area. They've already said that there will be 2 places to visit on Earth - a regular Y1 cosmodrome with no changes (I kinda hope that the Taken stay as hunting Zealots for exotics is fun) - The Plaguelands - which includes a winterised Cosmodrome, snow, a dead walker in the Divide, SIVA creeping in and all new space with Fallen SIVA infused enemies and structures. It seemed at first as if the Plaguelands would be slightly distant, but from the images we've seen with the destroyed section of wall it seems more and more as if the Plaguelands are directly adjacent to the Cosmodrome wall. It seems as if this revamped winter Cosmodrome is going to have new playable areas opened up on the other side of the wall, it's genuinely expanding into the surrounding terrain - and I don't think that the legacy consoles would be able to handle that. Especially not if we are going up on top of the wall with a massive render distance.