I'm only speaking for myself with the below. ymmv of course
I get the "grass is greener" feeling sometimes, so I go looking to see if anything better has come along.
Then there's just the practical switch. My main job is in .net right now, while my home computers are all linux distros. It's made me search through a lot of editors to find something that runs well and feels good with windows.
I also just like to explore software, you know? A for instance would be Sublime Text. I've said it in a few other threads, but I do dig some of the features in ST2. They really did multiple cursors right. It's a really simple, but intuitive and powerful way to edit things. Using that on ST2 me want it everywhere else, and also made me fall in love with how they set up the command palette. Using emacs for a bit made me really dig how elisp worked. Light Table showed me another take on the integration of a language into the editor itself. So it's a lot of curiosity with that, and to know what's out there.
As an aside, if Neovim delivers it's promises, it'll probably solidify vim even further as my only editor of choice, but I'll still go looking around to see what other people are coming up with.
If the analogy holds, the answer is the same reason people play different video games. I think the analogy holds a bit: variety and different kinds of mastery feel like a correct answer.
