Not the OP but Tcl comes with batteries included and more "full fledged". Lua is just so much more lightweight and so easy to just drop it in any codebase.
The biggest reason to use Tcl even to this day is to write Tk GUI programs.
The biggest reason to use Tcl even to this day is to write Tk GUI programs.
Well... Tcl on its own is good as a configuration language, creating DSLs, or as a scripting/extension interface, as well as a single common cross platform interface to filesystems, network sockets, and more - so I'd say "the biggest reason" depends on what you're working on...