The reason isn't a flaw in technology, because many alternatives to electron do in fact offer markup language like GUI creation and css/html like styling including Qt with its QML offshoot, or JavafX.

Not to mention that I don't see any obvious reason why HTML would be easier to use than the WPF / Swing / Glade Gui-builder paradigm. As a not web-developer, I certainly have less issues building a Gui the good old fashioned way than having to learn HTML.

It honestly baffles me that anyone would consider HTML to be a good fit for a complex desktop application. I am actually very glad that I can drag buttons on a canvas and click on elements to understand their apis and adjust settings visually. I am not excited about building complex gui's by interacting with plain text that was designed to put text on webpages.

The reason why Electron has dominated has a much more trivial reason. There are a lot of web developers, and the javascript ecosystem is so large that it allows the quick reuse of a lot of code, saving those developers a lot of time.