A few thoughts:
* Cost. Why hire a separate dev team for each of web, Windows, and MacOS (and in this scenario, you never get a Linux app) when you can hire a single dev team to work on the core of the app with maybe one person to handle platform issues (and you get a Linux app nearly for free, which you wouldn't otherwise bother with)? Hell, you might not even need to hire more people to deal with your desktop apps initially, if you have at least one web developer who is resourceful and can figure out the remaining details.
* Hiring. Web developers are everywhere and easy to hire. Finding someone with expertise in a particular GUI toolkit is harder. Even if you initially don't care about which particular GUI toolkit, after you start building in one, you need to find more people to hire who are familiar with it. (This is also another "cost" item, I guess.)
* Ignorance. My biases are probably showing here, but my impression of most frontend developers is that they don't worry about performance too much, at least not to the level that, say, a C++ or JVM or CLR developer would. They seem to view many performance issues as something to be solved by the Javascript engine or browser renderer. They also likely develop and test their app on a high end machine and don't notice perf issues until users complain.
* Marketing. Visit Electron's website, and then visit PyQt's (or Qt's, though they're a little better) or GTK's. I guess it shouldn't be a surprise that the websites made by people who work on web technologies look better and tend to have better marketing.
* Tooling. Web debugging tools have gotten pretty great during this decade, closing the gap with native-code IDE debuggers, gdb, valgrind, etc. Electron's tooling make it fairly easy to build for each platform. Building a cross-platform app in Qt or GTK requires native compilation for different platforms, which is still, even today, a bear to set up.