The reason I chose Vim over Emacs is simply because of the fact that it's available literally EVERYWHERE. I've honestly never seen a UNIX system which doesn't have Vi or Vim installed by default. Emacs is more hit and miss.
(source: vim user for twenty years, Emacs ten)
Chrome and youtube also support some of the common vi/vim navigation keys.
After using both I settled on Nano years ago and still don't see how my workflow would improve with Vim.
To me nano is just a basic text editor (open, edit, save). Vim is a way to 'jack in' and be able to manipulate text with my mind. I don't 'think' anout how to accomplish something, I just think about what I want done and my hands just do it.
This comes from the modal nature of the editor and text objects, mostly, but there are so many other little things (like macros, ex commands, filters) that you pick up over time that all add up to a great text manipulation toolkit.
That said, editors are a personal thing. Pico was certainly a better choice for most people. In most cases, their needs are basic. Pico also displayed the relevant keyboard shortcuts on the bottom of the screen. Why would they want to complicate things, and vi would complicate things for people who did not have a need for a text editor outside of composing emails.