I think most people do have a general idea to those answers though. Here's my stab at it:
1. They do know why its empty. Moving the server was the last straw, but there's a deeper problem with the game and IS that has no easy solution. It was discussed on US forums several times and nobody really came up with a perfect solution that didn't have large drawbacks. The best propositions was only a band aid slapped on, and not really a permanent, lasting solution.
2. They don't know how to populate it again. If they advertised, any new player going to the AU server is just going to be told by older players to leave because AU is in a state of @#$% anyway, so why bother advertising. This seems to be a common thing in RoM servers on US too. We complain that they don't advertise and get more players to play the game, but as soon as a new player comes on the forums and asks we instantly tel them not to bother playing the game, lol. I think its a bit better on US now, but it still happens a bit.
They're probably just keeping AU alive so that it doesn't cause panic in the other severs. Because as soon as they shut down AU a lot of people on other servers are going to think all servers will close soon and stop spending $$, so that even if server was doing fine before, people will literally cause their fears to become true.
3. Because the don't have confidence that they can do it w/o screwing up, and if they screwed up a US server for 10 people, it'll probably cause them to lose more.
4. To save money.