Currently? Zero reason.
Lummy, the creator of Gitea, wrote a post about why he moved Gitea over a company.
The TL;DR; is that it allowed Gitea to better work with companies that wanted to fund them but couldn’t fund individuals directly, moved the Gitea trademark to an entity that could outlive the creator, etc.
There hasn’t been any significant changes to how Gitea operates or can be used from a user point-of-view since the change happened.
Forgejo is a fork of Gitea that will pull changes made from Gitea, without being a company. In the events that Gitea goes to shit, it’ll be there as a replacement.