There is one reason. Among a number of other reasons I believe it's the sole reason for it and that it influences the other reasons.
Control.
Apple controls downgrading as a means to prevent jailbreaking, which in turn controls access to phone features that the user's carrier and plan have otherwise restricted, non-standard device setups and installation of non-Apple software that modifies iOS and thus deviates from the standard norm. It means less support issues (Apple doesn't troubleshoot jailbroken devices) and keeps the user base standardized within a few degrees of the same OS - which means solutions to user problems apply to a broader range of the user base.
And yes, some will argue that this control increases security.
It's that small surrender of a bit of control that drives me to jailbreaking. For Apple, it's that small gain of control that allows them to counter jailbreaking.