JS had nowhere the popularity it had today. At the time, creating a desktop app in Javascript was not regarded as serious. The reason electron became popular is because so many people from other fields (designers, integraters, students...) learn programming through copy / pasting JS and wanted JS for everything. Hence node. And now electron.
They didn't advertise that much, though. I only learned about XULRunner when I was trying out a keyboard-driven browser (Conkeror). Unlike Electron, XUL wasn't promoted.
uh, I wouldn't say electron got popular because of copy-pasting newbies.
webapp development is a lot of work and SPAs are actually even harder... and so is server development with node. I love javascript, yet I don't think it's suitable for everything.
electron got popular, because companies could reuse code and tooling, only for the cost of cpu & memory
