I think this is partially due to the informality that many GitHub users tend to have with respect to managing their projects.

It's rare to see a GitHub-hosted project that offers proper releases, proper planning for those releases, proper documentation, and the organization and project management efforts that enable such things.

The lack of such basic project management structure leads to projects that see very erratic development, or outright abandonment. Many GitHub-hosted projects are barely more than a hobby, rather than an ongoing concern. Their code may be somewhat usable, but it's rarely comparable to what we see from more organized projects.