I guess it's because gorilla/websocket has had a really big mindshare for a very long time.
and no project has had time to catch up and/or provide "reliable" hints that they will maintain that new project for a sizeable long time.
"the devil you know" and "the technical debt you know"...
My hunch would be that so much boilerplate already exists, and golang is so extremely popular, that a lot of it is being pulled into tutorials, samples and new projects. If it already exists there, I doubt the many users even check the repo.
Yeah, I think that might be why it keeps spreading even after being deprecated.