I see there a few problems:
1) github is possible because it’s key file type — code. Graphic files are typically very large, you will either delete everything old on a regular basis or pay a lot for storing a huge amount of files.
2) Service is closely connected to sketch. I am not sure that the same versioning would be possible with adobe file formats.
3) Team work over the projects… well, it’s not very common. The most of design work is made by a single person, so for what would I need such service? At the same time, team work is a key feature of developing any large software — that’s why git works.