Yeah, you would think it would not be that popular, but it's a historical phone, in that, to our knowledge, and regular peoples knowledge, (even though reported that Steve and Crew have 4yrs worth of stuff planned out) it's not only the phone that was released after Steve Job's passing, it is the last iPhone he has worked on. It is the fastest iPhone to date with much better specs than its predecessor. There is Siri that you can't get on the iPhone4 or 3GS. And there are people who still have the 3GS.
That's a lot of stuff and then there are other reasons too. A lot of people don't want to wait it out for the iPhone5 they want this one now but couldn't preorder. Some people's $ wasn't right for that but they get paid on Friday and that is when they could get it.
Put all those reasons, and more, into a barrel and they overflow to where people, a lot of them, get the same idea and camping out ensues. Oh yeah, then you have the people who get it on Launch day every year and a little rain isn't going to stop them.