Two reasons, at least for me.
1. Something different from regular road biking. It's fun, a nice change of pace.
2. Think of it as a nice road bike being a 7 series bmw with a 7 or 8 speed auto transmission. Its fast, usually comfy, has all sorts of doo dads and technology. A fixed gear is like a Porsche GT3; no A/C, stiff suspension, race transmission, etc. You have so much control over the bike. You are legitimately connected to every thing the bike does. I know its a lame cliche, but people say you're "one" with the bike. Once you learn to ride a fixed gear properly, which takes a couple years, you can absolutely haul ass everywhere and there's no other type of biking like that. Skidding around a corner at 20mph is fucking awesome.
Now the whole trend thing is just that, a trend. In a typical college town, 90% of the people on fixed gears are a safety hazard on the equivalent of target bikes.
Another reason college kids have single speeds or fixed gears is that they think they are low maintenance. Which they are, but only if you know what you're doing in the first place, which usually isn't the case.