My reason for vi is equally random. I learned Unix at college, but I wasn’t allowed on the systems because I wasn’t an engineering student. I had to resort to nefarious means to get online, so I needed to keep a really low profile. The easiest way to do that was to remote into unused boxes in the lab. (This was back when it was easy to get onto an intranet and hard to get on the Internet — it’s exactly the opposite now!) The systems that were usually available were these crappy diskless Sun workstations that booted over the network. They didn’t have enough memory to run emacs without paging and paging over the network is a recipe for frustration. Plus if someone logged in and the system was slow the first thing they’d do is try and find out why, which might lead to them wondering why I was on that system, just who I was, etc and that was a bad direction for me. I also couldn’t talk to people in the labs since I wasn’t supposed to be there, so I was entirely self taught, and emacs seems like the sort of thing someone has to evangelize to get you over the initial hump.
Anyway, that’s my story — what’s yours? :)