This isn’t a big deal for most consumers, but in previous Android versions, it was nice to have an escape hatch if Google came out with a particularly buggy first release. If you frequently try out different software builds, this change will presumably mean that you can’t use any older third-party ROMs, either.

Update: We have a justification! As spotted by Esper’s Mishaal Rahman, the Android web flash tool explains this downgrade prevention is to stop a bootloader vulnerability specific to Android 12.