What does Electron offer in terms of portability that SDL2 doesn't have?
With the disclaimer that I don't know a lot about this: Electron has solid support for both desktop and mobile targets. I don't think Qt's mobile support is as good, but I might be mistaken.
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.