It's only JUST NOW we have truly portable UI frameworks. And it's only because of the Web.
You say that the web is portable, but really, only Google's vision for the web is relevant, seeing how they have the final say in how the standards are implemented and evolved.
So it's basically another walled garden, only much bigger and not constrained to the CPU architecture and OS kernel.
Chromium IS a platform. And indeed many applications that do work on Chrome don't work on Firefox. So we're pretty much back where we started, but the problem is harder to see because Chrome has such a monopoly over browsers that for most intents and purposes, and for most devs, it's the only platform that exists.
Everyone is good at multiplat when there's only one plat.
Web apps are popular because 1) people don't like installing things anymore for some reason and 2) it's easier to justify a subscription pricing model.