If you ever want to be able make your own decisions, Android would be ready to help!
To prevent devices from being downgraded to older versions that lack the latest security updates, iOS uses a process called System Software Personalization. If downgrades were possible, an attacker who gains possession of a device could install an older version of iOS and exploit a vulnerability thats been fixed in the newer version.
What has that got to do with anything? The OP is talking about iOS.
But why do you want to downgrade, anyway?