> I'd be more curious as to why people are staying on GitHub

Vanity metrics.

GitHub initially tried to shy away from this, I remember conversations with early GitHub engineers trying to make sure "Stars" and "Followers" numbers were going into the direction of being just for vanity and popularity.

Then eventually the profile READMEs appeared, which people now use for showing even more vanity metrics and brag about how much code they can produce in how little days.

Since employers also ask you for a GitHub profile, it ends up being needed for new developers to make an entry into the industry, without it companies will basically ignore you. Unless you're really, really good, which to be honest, most of us aren't.