Why catalina doesn't support garage band again

The current version of GarageBand, version 10.4.3 has been written for macOS 11 Big Sur and can only run on the newer system, but you can work on Catalina with the previous version, that has been released for Catalina.

If you ever downloaded GarageBand 10 from the App Store, you should be able to reinstall the older version of GarageBand 10 from your list of previous purchases. Have you tried this?

Sorry, there are no official options to get GB if you don't have Big Sur.

You can't easily. Have you ever downloaded GB before?