Historical reliability and compatibility. They claimed they were S3 compatible, but they were requiring deprecated S3 SDKs, plus S3 advanced features are unimplemented (but at least they document it [0]). There was constant timeouts for object creation and updates, very slow speeds and overall instability. Even now, if you check out r/hetzner on reddit, you'll see it's a reliability nightmare (but take it with a grain of salt, nobody reports lack of problems). Not as relevant for DB backups, but billing is dumb, even if you upload a 1KB file, they charge you for 64KB.

At least with Storage Box you know it's just a dumb storage box. And you can SSH, SFTP, Samba and rsync to it reliably.

[0] https://docs.hetzner.com/storage/object-storage/supported-ac...