Because the archivist or publisher who controls the archive/zine you used decided to move to AO3. While all of AO3’s tools are designed to give as much control to the individual fan as possible, we believe that the archivist has responsibility and authority for the body of work as a collection. Archivists and publishers have moved archives onto different servers, updated the software that organizes the works, switched webhosting, reprinted zine issues, etc. to keep a collection of fan works alive, online, and together. Transferring or backing up a collection onto AO3 is just another way of ensuring an archive’s long-term accessibility. The archivist will also continue to have control over the collection if they want and can set its rules, etc. Nonetheless, if an individual fan wants to remove their works from the original archive or from AO3, we provide easy tools for doing so; see below.