Because it looks ugly (i.e. non-native) on pretty much every platform. Qt has different levels of success at that, having most problems with macOS and Android.
Also, there's Qt Quick, which lies somewhere in the middle between classic Qt and Electron.
>It's slow to render? Wrong, Electron (Chromium) renders faster than Qt. The Skia engine is optimized to an extent that these other toolkits cannot compete with. It even has a Vulkan backend now
The reason why classic single-threaded GUI toolkit are the way they are is because you can perform a custom reactive drawing or just reactive UI update in a blink of the eye, so user won't notice any delay. I know there are some bad examples like java-based IDE bloatware, but there are good examples like... Chromium! However, you cannot develop Chromium with Chromium, but you can develop something similar to Chromium with Qt.
>Memory? It's 70MB
Haven't seen Electron application using less than 200 Mb of memory. For example, launching Skype plus chromium SPA takes 550 Mb of RAM on my system. JavaFX has similar problems.
>Slack is a slow monster, but it's a giant codebase that does things that would be simply impossible in Qt, GTK, Swing and others
Like what? I'm not aware of any special Slack's features that could be hard to implement using classic GUI toolkits.