Most likely due to network restrictions, or certain helper software not available. CPAN has sometimes a hard time getting out of a firewall (which is sometimes caused by the person configuring CPAN not knowing what to answer to all the questions). Also, to get out, CPAN is geared towards LWP - which - *grumble* *grumble* isn't part of the core. And where getting out with LWP might be possible, it won't be possible with ftp, or whatever CPAN tries if LWP isn't there.

But as you said, downloading it yourself and running perl Makefile.PL, make, make test, make install is fairly simple. I don't know about other GUIs, but Tk doesn't have dependencies. It takes a few minutes due to all the compiling, but installing Tk is simple. In fact, it's easier to install by hand than from CPAN - if you install from CPAN, you compile/test running as the same user you're going to install the modules with. And that's not necessary the same user as you're normally running under. Which, at least in my setup, means that the user making/testing Tk doesn't have permission to display widgets on the screen. Resulting in a test failure, and an aborted install.