What's replaced Tk for an easily scriptable, cross-platform GUI language? What I see the most in practice is PyGTK, but it's pretty hard to love. Javascript+HTML is probably the most capable alternative but the lack of a simple container for native-feeling apps is really limiting. I was surprised to learn Lua has quite a robust series of GUI options, but I've never seen it actually used.
Like mechanical_fish says, lots of people have migrated to the web. Others use Qt, although it has some licensing limitations, or used to at least. Plenty of people interested in getting stuff done still use Tk, be it with Tcl or some other language, quite successfully. It does look better these days.
What really happened was that the core functionality of Tk was ported to other languages (for python, check out Tkinter), so you can leverage the power of Tk elsewhere. As a result, you have to look at Tcl in comparison to Python and other languages. At the end of the day, Tcl fell out of vogue.
If memory serves me correctly, Tkinter even exposes you to Tcl sometimes if you want to do more advanced things, though I can't remember now why I needed it.
