It was commented out in

commit 43ff8683fe68424b9c179ee4970bb865e11036d6
Author: Guido van Rossum <guido@python.org>
Date:   Tue Jul 14 18:02:13 1998 +0000

    Temporarily get rid of the registration of Tcl_Finalize() as a
    low-level Python exit handler.  This can attempt to call Python code
    at a point that the interpreter and thread state have already been
    destroyed, causing a Bus Error.  Given the intended use of
    Py_AtExit(), I'm not convinced that it's a good idea to call it
    earlier during Python's finalization sequence...  (Although this is
    the only use for it in the entire distribution.)

Maybe Guido remember more details about reproducing a Bus Error. Was it an issue from the other side, when the Tcl/Tk interpreter was used after calling Tcl_Finalize()?