alt Hacker NewsAntirez (the author of this interpreter) uses it for the Redis test suite.
Personally, I know Lua and Python very well but I still used TCL a few years ago for something very specific: using ODBC on Windows. I gave more details on the Lua mailing list here: http://lua-users.org/lists/lua-l/2021-01/msg00067.html
That was interesting to read. So in your case it came down to ecosystem depth, especially on Windows - where there was a TCL solution to talk ODBC, but the equivalent Lua functionality had issues on Windows.