A little of column "A," a little of column "B." The older content was being played by only a small percentage of players, yet it was taking up a disproportionate amount of game resources, as well as time spent updating that content to keep it working with each new content drop. Vaulting the older content allows Bungie to focus on new and current content, while also potentially improving the underlying tech running the game without having to worry about breaking the old stuff. Story-wise, just before the servers shut down on November 9th to start rolling out the updates for Beyond Light, there was a live event where the Traveler began repairing itself, and in response, the Darkness consumed the vaulted planets. Presumably, those locations haven't actually been destroyed, and at some point, we'll find out more about what actually happened to them.