> Why shouldn't it be Electron?
Because every Electron app is inconsistent with the rest of the desktop. I use a dark theme system-wide but Electron won't care [edit: 1Password has custom integration for GTK theme]. Honestly, this isn't something the developer of the app have to put years of research in (Slack for example). The toolkit is supposed to do the integration (GTK, Qt, [Cocoa?]) and clearly Electron doesn't care.
> Why not QT?
You tell me (assuming you're talking about Qt, not QuickTime)
> Electron doesn't seem like a worse choice than the other options
Not really. Its just that its lazier/cheaper to just get your web development team pretend to write a desktop app. I get it, business decisions need to factor cost into account and hence the choice. I understand when a business says "we just don't have the funds to use a proper app framework, please do with what we have for now". But instead everyone goes to pretend like Electron apps are perfect even though the reason it was chosen was almost completely based on cost.