I'm Running on a Late 2009 27" iMac Using High Sierra Version 10.13.6 Why does the New OS not work with my setup!

Your mac does not meet the hardware requirements for the newer OS's.

When Apple moved to macOS Mojave and newer macs need to have a graphics card that uses

the 'Metal' API, which your macs graphics card doesn't, other things might be WiFi cards, bus controllers etc.

Also Apple doesn't really want you upgrading older machines indefinitely.

They want you to go and buy new macs.

Probably because, even the late released version of this computer from 2009 can only run High Sierra and nothing newer in the way of macOS.

Which new OS???