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.
