I have an imac from late 2013, why cant I upgrade to the newest IOS?

Your iMac doesn't meet the minimum hardware requirements macOS - How to Upgrade - Apple

You can run macOS 10.15 Catalina How to get old versions of macOS - Apple Support

Because it is too old. It is not supported anymore.

Cheers,

Jack

Apple has made 21.5 iMacs since late 2009, would you care to elaborate to tell us which year 21.5" iMac you own?

I have a 21.5 inch iMac and the IOS is Catalina. Is there an upgrade that I can instal?