Because Electron provides an awful UX. It ships with an entire web stack for applications that could be 1/100th the size and 100x the performance if they used a native toolkit instead of a web view. Electron apps also get none of the native L+F that you get if you use an actual toolkit properly. Electron is to the desktop as hybrid apps are to mobile.
I don't think that's true. You can achieve a lot with just react, css, node and the electron APIs. With enough effort and attention to detail you can make an app look and feel native with css (https://www.youtube.com/watch?v=JIRXVGVPzn8), V8 is not /that/ slow, and you have the advantage of using multiple node processes inside electron to offload work. And it's cross platform.
Yes, the built apps might end up being big, but that's a trade off I'm willing to take in favor of developing cross platforms apps with a modern and familiar web stack.
The obvious advantage to native look and feel is that it will be familiar to users, and that consistency in itself might make for an overall more beautiful look.
If I use 5 applications and they all look entirely different, it will not look beautiful, and each time I swap to a different application I have to re-familiarize with whatever the UI designer thought was a good idea for that application.
Author here. In most cases, Electron is much better in many ways.
I'm working on the code editor right now, so I'd like to build something small (few megabytes), performant, but allows me to use the whole js ecosystem.