Accessibility would be my immediate thought, although I'm not massively experienced in terms of GUI programming, so I could be off the mark here.
With something like SDL2, I would have to interface with each operating system's accessibility API directly, in such a way that's not easily portable, unless I bring in a separate library. Even something like GTK has this problem on platforms other than Linux. With Electron, I know that my program will be accessible wherever Chromium's rendering of HTML is accessible, which is a lot more places than anything I'm going to be able to bodge together.
With something like SDL2, I would have to interface with each operating system's accessibility API directly, in such a way that's not easily portable, unless I bring in a separate library. Even something like GTK has this problem on platforms other than Linux. With Electron, I know that my program will be accessible wherever Chromium's rendering of HTML is accessible, which is a lot more places than anything I'm going to be able to bodge together.