Not defending Electron, but Cocoa is an OS-specific, proprietary framework. It makes sense why most text editors (which are mostly libre software) wouldn't use it. I do have to ask, though: why not just use TextMate, at that point?
That's one reason exactly. TextMate is a phenomenal piece of software, and I've used it for years. Will continue to use it probably. In many ways, it was an inspiration for Chime. It can do many things Chime cannot. But, TextMate does not offer the same language-level support, syntax highlighting accuracy and speed. And, I like to think Chime has a more refined UI, but that's a personal thing :)