Makes me wonder, are there any current projects still using tcl? I know DDG is still using perl.
The main problem is just tooling. There are some insanely cool language features like Expect (still the best implementation) or starpacks/starkits, yet there is no basic package manager. You are stuck installing via yum/apt or building from source if you are lucky to find what you need. Most of the time you can just assume there isn't something you can rely on, and you'll have to patch source tcllib (the batteries included library).
Some components of the GNU toolchain use dejagnu in their testsuite, which is based on expect and written in Tcl.
The SQLite testsuite uses Tcl, too.