It's old and primitive, but it works with minimal headaches on pretty much every platform you can think of

Platforms I immediately think of: the web browser, tablets, smart phones, JVM. Granted, Perl doesn't run on them either (but that's something Jesse would like to see fixed), but thinking "pretty much every platform you can think of" are Windows/Linux/Other Unix/Mac OSses running their GUIs is very 1990s. The world has moved on. "Platform" nowadays is so much more.

I don't know any platform that runs Perl/Tk, but doesn't run a graphical web browser. But I bet I'll see dozens of platforms fully capable of running web browsers, but not Perl/Tk in my walk from the office to the train station.