Developers really only need to share code with other developers. There's almost no such thing as a passive receiver of code. The ability to easily collaborate on code with other developers and within a team, via cloning, forking, pushing and pulling, makes Github (and Bitbucket and Gitlab, etc) immensely beneficial.
Design interactions, on the other hand, tend to be made up primarily of designers sharing designs with non-designers, whether it be the client for feedback or the technical team for implementation (or the printers, or the marketing department).
I think Pixelapse is more like the "Github for design" if you just think of Github as a 3rd-party repository for easy version tracking and uploading of code, and which makes it easy for others to comment and compare different versions of the code. In other words, minus the forking and cloning concept. At least, that's how we use it.
Open source design could be massive. There are already tons of places where designers give away assets to the community - fonts, PSD templates, logo templates, etc. But they all have the limitation of being done by a single designer. I think - despite the fears of "design by committee" that some might have - there could be some great stuff that comes out of collaborating in this way.
The true social value of Github is in its community-oriented approach where any developer anywhere can fork code and contribute changes. That doesn't seem as possible with imagery, and my impression is that designers are more concerned with protecting their work (nothing wrong with that - just an observation).