Apple iPhone iCloud Deleted My Old Photo Backups Without Explanation

I currently have an iPhone 8, and before that, I was using an iPhone 6, where I made sure to back up all my photos prior to the upgrade. I decided to buy extra iCloud storage because my current space was full, and I wanted to make sure everything was secure, so I completed a backup. To my astonishment, all my previous backups disappeared without any reason given. I find myself questioning what happened to my cherished memories—photos that held great significance for me. Where have they gone? I had faith in Apple's system to safeguard my data, but now I feel disappointed and unsure about depending on iCloud moving forward. I’m looking for some clarity on why my old backups have vanished and what steps I can take to recover them, as losing these photos has been quite distressing.