Judging from TFA it's due to KeePassX having one maintainer, so the pull requests all bottleneck with him. Also mentioned is that one desirable outcome of this is that KeePassXC developers are given co-maintainer status of KeePassX and that they re-merge down the line.
That's cool. But if that's all this project is, couldn't they have just talked to the one maintainer and offer to help him and be on the team?
I would assume that avenue has been tried -- so this project is to pressurise him into opening up to the idea. No idea though.
I wonder how often those conversations take place before someone(s) fork a project because it's "abandoned".
But I love KeepassX and I'll certainly try using the fork.