Java didn't only run on one OS but for whatever reason Oracle seems to find no value in making its UI stack nicer and modern. It seems only Microsoft's C# stack is working on this. There's also Qt, which is not a single-OS solution, but its C++.

Well, there is JavaFX, which is really nice for modern java desktop UI's. However, Swing can be taken really really far - just look at IntelliJ and friends (most people don't even realize they're Swing UI's).

With that said - most modern applications are webapps for good reasons. Making native apps sucks for a lot of reasons - including all the random OS-specific behavior you have to work around, specific versions of native OS libraries, etc.

Building for the web browser means, without any extra effort, you app works on all operating systems, and it works exactly the same. That's a pretty good sell to anyone trying to make a modern application that's mostly just a front-end UI for an API...