Most Chinese sites do not use HTTPS. In fact, TLS 1.3 traffic seems to be completely blocked within China's internet.[1] The decision to use plain HTTP is only strange from a Western viewpoint. Note: I am not defending this behavior. I still remember the era of ISPs injecting content into webpages. But it's important to keep in mind our subset of the world does not reflect the rest of the world.

[1] https://news.ycombinator.com/item?id=24093932

It does reflect the rest of the world; China is the extreme outlier here.

Also, accessing GitHub from within mainland China works, so TLS is not completely banned.