In my case, travelling a lot was a big factor. I've always been open minded, and like to try new stuff. I found out that there's a whole world of good beer out there, and the idea of being able to make it myself was just too cool to forget about. I think that's probably a contributing factor for many military brewers. Plus, I think a lot of military folks have more of a "To hell with it, let's do it" kind of attitude.
As luck would have it, a certain co-worker of mine just happened to get a kit for Christmas, as did I. So, we went into it together. If it hadn't been for Dude, there's a good chance that original kit would still be sitting out in the garage.
Also, work schedules are different for everyone. Me and Dude do a lot of rotating shift work (him more than me), so some weeks you have a nice bit of time off, others it seems like you do nothing but work and sleep.