Bungie can't do large maps. There is something about with their basic software that prevents an open world. Everything is tiny map sections connected by narrow roads or tunnels that allow the next section to load. There is also a limit on the total of all maps put together, which is why they had to remove The Tangled Shore before Witch Queen launched. There isn't room for Mars. They won't invest in modern open world software, so we're stuck with losing one map in order to gain another.