I actually know the answer to that one!
> The principal lesson of Emacs is that a language for extensions should not be a mere "extension language". It should be a real programming language, designed for writing and maintaining substantial programs. Because people will want to do that!
> Another lesson from Emacs is that the way to make sure an extension facility is really flexible is to use it to write a large portion of the ordinary released system. If you try to do that with Tcl, you will encounter its limitations.
> Tcl was not designed to be a serious programming language. It was designed to be a "scripting language", on the assumption that a "scripting language" need not try to be a real programming language. So Tcl doesn't have the capabilities of one. It lacks arrays; it lacks structures from which you can make linked lists. It fakes having numbers, which works, but has to be slow.
From Why you should not use Tcl by Richard Stallman [1].