This sounds like your wallet encryption password was stored in a file on the storage device for your Bitcoin Core wallet. If you made backups of the wallet file, then why didn't you make backups of the password file? This seems not very logical to me.

Who created the wallet encryption password? Was it human generated or some random password? Humans tend to have certain recipes how they "invent" their passwords or they re-use common passwords they had and make only little adjustments.

Knowing the details about possible password usage may help a lot to crack such an unkown password, but you need to have at least some ideas about what could've been part of your to be cracked password.