Because the archivist who controls the archive you used decided to move to the AO3. While all the AO3’s tools are designed to give as much control to the individual fan as possible, in the case of large archives we believe that the archivist has responsibility and authority for the body of work as a collection. Archivists have moved archives onto different servers, updated the software that organizes the works, switched webhosting, etc. to keep a collection of fan works alive, online, and together. Transferring or backing up a collection into the 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 stories from the original archive or from the AO3, we provide easy tools for doing so; see below.