I see absolutely no reason why one would pay GitHub for private repositories when there is Bitbucket, or much better alternative to GitHub altogether - GitLab.
We switched from GH:Enterprise and saved $1000's a year. Worth it.
Because businesses need money to survive and most have no problem spending it when it increases productivity or provides perceived value over another product.
When people mention GitLab I always wonder if you mean the public hosted GitLab, or running your own GitLab.
Usually, they are referring to the public GitLab as it is a free installation of GitLab Enterprise Edition with free private repositories.
Public GitLab is amazing, especially built-in CI integration, however it is not as fast as Github (page loadspeeds)