The main reason (JVM apps are not more popular) is that the average JVM desktop app does not perform better (or use less resources) than the average Electron app. I know this is not supposed to be that way, but one way or the other, it is.
On top of this the web UI developer community is orders of magnitude larger.
I don't think that's really true. Years ago, I maintained a Java desktop application, using SWT as the graphics library (not even mentioned by the post author which is weird.) It wasn't a big application, but also not stupidly trivial. It would start fast, run fast, and didn't need much RAM at all.
There are of course examples of slow, heavy java applications, but I don't think it has to be that way. I'm not totally sure what I did differently aside from consider my dependencies (e.g. using a lightweight dependency injection framework rather than something like Spring), but it's definitely not a given that Java desktop applications have to use a bucketload of RAM.
