The project had already forked in the past, but it eventually was deleted and was merged into the upstream because of one very simple reason: Unknwon, the creator of Gogs, came back. The fact that he left again is the main reason why the project forked again.

It's not like other people are mentioning in the thread that "some contributions would not get added" - but rather the fact that he often has really long periods of absence: just take a look at the contributions on his profile https://github.com/Unknwon

And of course, I'm not putting the blame on him - all of us need breaks from time to time - but during these periods where he can't work on the project, the project is essentially brought to a halt, seeing as there is no one else in the community of contributors who is able to merge pull requests - even if they are critical.

Gitea is however the more active (and quite active) side of the fork so I'd say it was quite successfully. You can easily check this in the contribution statistics on Github for both projects.