Because Nick merged the Tcl/Tk code into Perl/Tk very well. There's a lot of Perl support code around the Tk core to make the Perl interface nicer. For instance, the Tk callback system is entirely customized for Perl and the object-orientedness of Perl/Tk is completely absent from Tcl/Tk. So instead of having to find an appropriate version of the library your toolkit needs, it's already been integrated into Perl/Tk. Something like Wx is still very C++-ish, which is fine if that's what you want. But I think that by including a particular version of Tk in Perl/Tk and then making a heavy Perl wrapper around that, the code becomes more usable and the install a little simpler.