My guess on the actual reason it's not day one, is that there are so many changes that they want a chance to see how the community plays and look for ways that "unexpected results" could influence the raid mechanics... Give them some time to make sure sure it will play as intended.
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This I can kind of understand. I think a reason is they are waiting for the community to find the new best guns, then nerf those guns along with the drop of the raid so that it's more difficult by default.