To me the rationale behind it (the answer) is, the timing (synchronicity) between the two events.
What´s the use of having both happening at the same time? that is, if the hurricanes is passing by and you are still moving people to the shelter, you and the people you are supposed to save are in danger.
If you say that "people are being moved" until the hurricane passes by. It suggests that you are still moving people. Perhaps before, during, and until it passes by.
Hence, "all the people in the neighbourhood have been moved to a shelter until the hurricane passes by", suggests now that people were already moved, are sheltered right now.
To me it´s a more sensible thing to do than the former one, don´t you think?