Tk is a windowing mechanism. CGI is an interface through which you can execute server-side code to dynamically output HTML that gets rendered client-side. The dynamic HTML output by a CGI script ends up being rendered in the client-side web browser. Web browsers know how to render HTML, Javascript, and with plugins they can handle things like flash. But they don't do Tk.

Think about it this way. Your CGI script executes on the webserver. If you create a Tk object within a CGI script, it is created on the server side, not the client side. It's rather useless to be creating Tk windows on your apache webserver. ;)