Qt is nice but costs money for developers of proprietary software. Electron has its own costs but most of them are externalized to the user. So it wins out.
? It's LGPL, so as long as you link dynamically, it's fine for proprietary software to use it. A lot of commercial proprietary software in the VFX industry does this (Nuke, Katana, Mari, Maya, Houdini, etc).
I guess there will be people going to say that QT is LGPL, which allows people to use in proprietary applications. But I think there must be people who just want permissive alternative, and Electron is MIT, so it is indeed more viable (liceise wise).