Because the Updater App is not reliable and broken since ages. You see that with the issue you're currently facing. It even needs manual patching for some other issues (https://central.owncloud.org/t/patches-for-known-update-issues-from-version-9-0-4-9-0-5-9-1-0-9-1-1/3549).
In the time you have looked for a solution and posted here the manual update should be already finished.