eM Client updates like the latest 9.1 version on the release history page are “not normally automatically pushed out” as with all interim updates “just incase there is any issues that need to be resolved”. So that’s why its not available yet with the check for update. Shouldn’t be far away though from being automatically pushed as per it seems pretty stable and no issues from what i can tell using it on Windows and Mac. So in the meantime just manually download it and install it manually with eM Client closed.
Note:- Backup eM Client first before upgrading via “Menu / Backup” if you don’t automatically do periodic backups.