Why is there no lock degradation in “synchronized” design?

Java SE1.6 introduces "biased lock" and "lightweight lock", so there are four states in Java SE1.6 lock, no lock state, bias lock state, light weight lock state and heavyweight lock state, It will gradually upgrade with the competition.Locks can be upgraded but not downgraded, meaning that partial locks cannot be downgraded to partial locks after being upgraded to lightweight locks.Why is there no lock downgrade design?