Tcl and Tk are trailing-edge tools that are out of fashion at the moment.

Nonetheless, they have some nice qualities and capabilities. And the fact that they were designed in the 1980s/1990s means that they're relatively compact and efficient (especially compared to something like Electron.) I like Tk (and Tkinter for Python) quite a bit and find that you can build some nice and easy to use interfaces with it.

I also still enjoy the retro idea (which Apple and others seem to have abandoned) that UI elements should be visible and discoverable.

I think it's the Tcl part that's more interesting. Like how Wings 3D is written in Erlang.

Because some people think that web UIs lack basic UI development principles and are a pain to use.

Framework name makes the story interesting.

because it's interesting? lots of programmers on this site!