We haven't gone back because there isn't a reason to. There's little new to test that we can't otherwise test in simulated moon gravity, and no real discernable resources worth harvesting. We can't stay long-term yet, so any and every trip is expensive as hell for a very short visit. It's close enough that we can use optics to observe it in detail, and many types of scans can also be done from planetside. Going back right now takes a lot of money, a lot of risk, and there's little reward to be had. We'll go back when we're ready to settle for longer periods of time.
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