They're using the same software (Forgejo)

More hosts better than one. I doubt codeberg will be able to sustain the load remaining as free as it is now when it will be popular enough that every cs student will be using it for their assignments (forgetting to gitignore their IDE cache files obviously). Or some smartasses start abusing it for free storage. I mean everything that github has to deal with currently.

IIRC their tos says it must only be used for opensource software (i don't remember if it was enforced yet but expect that they can remove private repos at any time or make them paid feature). Same story with sourcehut

Not all projects might be open source if they've got tools used by the police, military, etc. Codeberg doesn't allow repositories that aren't open source.