This is probably largely related to the behavior of their users, though: Github's interface is not a lot more informative than Sourceforge's if no one writes a useful readme, and there have frequently been well-written projects hosted on Sourceforge that have lots of good information on how it works, or how to use it. However, I rarely seem to hear about new projects at Sourceforge, and frequently do see stuff at Github that has well-written introductory documentation accompanying the sources.
I frequently want to read about projects that solve interesting problems, but might not have the interest in installing or using it. Moreover, many people seem to now be using Github as a "Host my interesting document easily" host, which reinforces the "I go to Github to read code / read about code" perception. I'm sure a large part of it is confirmation bias, since most of the interesting code-related things I read about here are hosted at Github. ;)
I don't think it's that hard to see that this is still preferable for many projects.
http://gcmakefile.cvs.sourceforge.net/gcmakefile/
Seriously? That's the best they can do?
They have different ways to present code with different design targets and different problems. I prefer GitHub for reading code online without having to check-out, I prefer SF when already working with a project where I have the code locally and using the online interface to hunt for changes.