I dunno, some of these are a pretty big deal, in particular:
Gitea repeatedly makes choices that leave Gitea admins exposed to known vulnerabilities during extended periods of time. For instance Gitea spent resources to undergo a SOC2 security audit for its SaaS offering while critical vulnerabilities demanded a new release. Advance notice of security releases is for customers only.
Gitea is developed on github, whereas forgejo is developed on and by codeberg, who use it as their main forge (also mentioned on that page). Someone dogfooding gives me more confidence in the software.
I think the reason is because apparently a lot of people are unhappy with a deal Nix inked apparently with a company that does business with the US' Immigrations and Customs
I like #Nix, I do not like what has happened to it.
With no explanation of what happened, the conclusion is almost certainly Internal politics.
It seems like forgejo split from gitea because it looked like gitea was going the route of gitlab. Idk if NixOS is going to commercialize though. Based on recent gossip it sounds like they're overly adverse to commercialization. IE banning people for having DoD connections. Aux's talk about special interest groups makes it sound like they're going embrace that like redhat.
As a side note, I understand why Gitea and Forgejo went for a "copy GitHub Actions" approach to their CI, but man do I wish more self-hosted repo software tried to copy Drone/Woodpecker instead. Iterative containers in the pipeline is such a smoother build experience, and it kind of sucks that Gitness is the only one doing it (that I know of).
There's plenty, but I do not wish to hijack this thread, so... have a look at the Forgejo 7.0 release notes, the PRs it links to along notable features (and a boatload of bugfixes, many of which aren't in Gitea). Then compare when (and if) similar features or fixes were implemented in Gitea.
The major difference (apart from governance, and on a technical level) between Gitea and Forgejo is that Forgejo cherry picks from Gitea weekly (being a hard fork doesn't mean all ties are severed, it means that development happens independently). Gitea does not cherry pick from Forgejo. They could, the license permits it, and it even permits sublicensing, so it's not an obstacle for Gitea Cloud or Gitea EE, either. They just don't.