It's because your computer can't tell the difference. c-[ and ESC send the same key-code. Same with c-i and tab. What you can do to get around this is to remap one of them to something like f20 using Karabiner, then map f20 to what you like in vim.
Can recommend interception-tools mentioned in that SO link. I have been using them for quite a while, remapping caps to escape in desktop environment agnostic way.
