Because those are companies dominated by webdevs that can't be bothered to learn anything else.
Even VSCode is probably a side effect of Microsoft trying to be cool and cattering to the new generations that don't know anything else.
For me, it just made me buy a Sublime Text license instead.
You could argue that it's a similar story for things like Slack - for business reasons, they more-or-less needed to have a web-based UI. If you're already committed to that, then it makes sense to use Electron in order to maximize your code reuse.
I can't speak to Discord or Atom. Those ones make less sense to me, but there might be a reason there, too.
Speaking completely personally, Qt has never felt like a viable option for a commercial project to me. It does have some bindings for other languages, but, at least last I checked, they're all community supported. When you're talking about language binding libraries, it's reasonable to think of "community" as a polite way of saying "poorly". That can quite justifiably influence a person's willingness to rely on them for non-hobby projects.
I don't like JavaScript much better than I like C++, but at least I can more easily think of business cases for why I might accept being locked into JavaScript. The ones up above, for example.
Electron exists because of Atom (yes, in that order). Electron was previously called Atom Shell[1] because it was created specifically to house Atom, although they mention it wasn’t created just for that. If there’s one app that has a reason to be built on Electron, it’s Atom, because it was an experiment to see how viable that solution would be.
But even they understand that solution wasn’t the best because it’s slow, so they’re experimenting with a model where the application logic is written in Rust and only the UI is built with web technologies[2].
Electron is good for an MVP if you have or need a web app. Companies keep using it afterward because it commodifies developers and externalizes costs.
I doubt Microsoft (VSCode) is in that camp. Or that that was the main concern of the others. And I think that's a facile explanation anyway: the same kind of companies flocked to do native mobile apps in the past -- but nobody much cared for QT on desktop still.
Otherwise I'm not sure why most people care about the internals of their editor.
They had a native app. They decided to throw it all away.
Tell me about it!