If you know both HTML and a proper UI toolkit, you'll see how insane it is to use HTML for anything else than marking up text. It's like launching a space shuttle to go to the supermarket around the corner instead of just walking. The only thing HTML has got going for it is how easy it is to show 'Hello World' on screen. Anything more complicated than that and it becomes a total horror show, especially compared to how easy to use and simple regular UI toolkits are.