For me, the reasons are varied. One is my love for science fiction, in all forms. Aurora is just an excellent example of a great big SF sandbox, which also happens to be after my own heart. Who wouldn't want to be able to conquer and rule over a vast galactic empire?
More practically, Aurora's the only game of its kind I can play. I happen to be totally blind, and while there are many multiplayer 4x-style games online which are at least somewhat accessible, there's a distinct lack of single player offerings. If I want a game of MOO, I have to find a friend willing to read the screen to me and let me play by proxy, not always practical. With Aurora, massive complexity aside, I can do it whenever the mood strikes me. The game wasn't designed to be playable by the blind, but thanks to some quirks of fate and the good will of Steve, it is. I can't stress enough how nice that freedom to play whenever I care to is.