Comment by lilyball

When making a private repo public, there's a high chance that there was stuff in the private repo that isn't necessarily ok to make public. It's a lot easier to just create a new public repo containing all the data you want to make public than it is to reliably scrub a private repo of any data that shouldn't be there.

More generally, you probably want to construct a new history for the public repo anyway, so you'll want a brand new repo to ensure none of the scrubbed history is accessible.