From a purely macOS perspective, the major issue (apart from JSON being slow in Swift) was that there's no good native editor control for source code. There's NSTextView but it doesn't scale to large, syntax highlighted documents and lacks the required facilities that a source code editor needs. Hence, all proper third party text editors for macOS write their own source code control. I, personally, would contest that a native GUI is very much possible, but only for the chrome of the editor (e.g. searching, menus). This is also where native GUIS can shine. I'd wager that a proper source code control is such a domain specific requirement that each cross platform editor has to implement them whether they use native UI or not (maybe Qt has a fantastic one, I don't know)