I think there's a few reasons. Although security may be the top one (including preventing jailbreaks), I mostly think it's because rolling back software in general creates a compatibility nightmare.
Apple devices are more than just an IOS system image. In cellular devices there are baseband firmwares that only newer versions of IOS support. The Touch ID SEP has a firmware too. When flash memory chips are replaced in new device revisions, those may need a certain version of IOS (I'm thinking about iPad 2s for example).
When IOS 7 came out, activation lock was introduced. If it was always possible to roll back to IOS 6, that would make the protection useless.
I agree with
@Wildkraut's scenario. There could be all sorts of unexpected side effects and crashes from a downgrade that Apple would have to deal with.
Lastly I also agree with
@C DM in that keeping as many people together on the same version of IOS helps developers, and hopefully keeps almost everyone on the same page.