There's a wide variety of reasons, so I can only describe my own, but it comes down to supply chain risk.

Just as with the COVID supply chain problems reconfigured trust and assumptions, the actions of Microsoft and other large US software companies have become erratic and untrustworthy. In particular the sanctions against the ICC prosecutor in Europe have made it imperative to have zero dependence on Microsoft with easy exit paths at any given moment.

Similarly, one cannot trust Google as they can disable your account one day, with zero recourse, zero ability to appeal, zero accountability to Google for disabling your email and SSO. Except maybe posting on HN and causing enough bad PR to get Google to act to suppress the bad PR.

The US is no longer a nation of laws, it's a nation of whatever the president wants, and even impartial courts are likely to be overturned by an extremely partisan Supreme Court that has said that the president can not break the law while doing "official acts", whatever the heck that means.

Audience is not a factor for me at all, if GitHub is a social network for you then it definitely makes sense to use it still.

But I don't know anybody that needs the social aspects of GitHub. That's just the people I know and work with, however.