Probably because Tcl has other redeeming qualities.
Command line programs are now passe in all except a few niches like routers, chip design etc, but IMHO there still isn't a better "/bin/sh++" that you can embed as command line shell interface between your C functions and the user.
I would make a distinction between a CLI (to be used by ordinary users of the program) and a scripting interface (to be used to "extend" the program, to be used by power users and/or other programmers).
And I would further submit that the same language may not be the right choice for both tasks.
Tcl leans further towards being a pleasant CLI interface language, while AFAIK Lua leans further towards being a pleasant plugin development language.
Command line programs are now passe in all except a few niches like routers, chip design etc, but IMHO there still isn't a better "/bin/sh++" that you can embed as command line shell interface between your C functions and the user.
Swig + Tcl + C/C++ works like a dream!