The main one is the concept of composability [0] - in vim, you're speaking a mini-language. You have commands (delete, copy, make uppercase,...), and a number of movements (right, down, to the next occurrence of a word, to line 27,...) or objects (word, line, paragraph, html tag, text within quotes, ...) to work on. These can be combined in any way you like, any command can be applied to any movement or object. So if you have a plugin like vim-angry that adds a "C-style function parameter" text object, all the commands you know can be applied to that. In contrast, in emacs, you have a number of commands (kill-line, kill-word, ...), and that's it. You have to get by with those. If they didn't implement the command you'd like to use, tough luck.
A second (minor) reason for me is that I work in the embedded software world, and usually on an embedded linux, there's at least some variant of vim or vi available, while emacs is very rare.
Take my advice with a grain of salt : I've never really used emacs, so I can't compare from first hand. From what I've heard, emacs is also a very good editor. To me, the question "should I use vim or emacs" is more or less the same as "should I use firefox or chrome" - I'm a chrome guy myself, but I don't consider firefox to be crap, nor its users to be idiots. Internet explorer on the other hand,...
So my advice : learn vim. And if you don't, learn emacs. Just learn to use either one properly, and you'll be way faster at editing than people who don't use those.
[0] a.o. https://medium.com/@mkozlows/why-atom-cant-replace-vim-43385...