Because you can not install an earlier version on the top of the current one. If you start to directly install macOS after downloading the installer, it won't succeed. Mac models compatible with OS X EI Yosemiteīack up Mac and erase Mac before downgrading macOS.Mac models compatible with OS X EI Capitan.Mac models compatible with macOS Sierra.Mac models compatible with macOS High Sierra.Mac models compatible with macOS Mojave.Mac models compatible with macOS Catalina.Mac models compatible with macOS Big Sur.As for Apple Silicon Mac with macOS Big Sur installed when leaving the factory, only macOS Big Sur and later versions are compatible due to the transition from Intel x86 architecture to ARM architecture. If you bought a Mac shipped with macOS Catalina, you can not reinstall macOS Mojave on it. The earliest version of macOS your Mac can run is the one that came with the Mac. In the first place, you need to check whether the earlier version of macOS you want to install supports running on your Mac.

Restore MacBook to older OS via a bootable installer Revert Mac to original macOS in Recovery mode Install older macOS With a Time Machine backup Back up Mac and erase Mac before downgrading macOS Thus, you want to install an older version of macOS on your Mac. However, the latest macOS Monterey or the currently installed one may fail to satisfy you, such as the Mac runs slowly after update, it is not compatible with the software you prefer to use, etc. It is recommended to keep your Mac updated to the latest version of macOS to maintain the security, stability, and compatibility, improve the performance of your Mac and experience the newly added features.

How to backup mac in recovery mode