I'd imagine the reason outside peripherals aren't allowed is because they present a security risk. However, even without that there's also liabilities. Like how SE can't be held liable if your peripherals are lost, damaged, or stolen.

In the case of flash drives, were they allowed then people could casually attach a drive to a computer and use it to record keystrokes, which they could then use to learn the usernames and passwords to the event accounts. That's just one example of why I think they won't be allowed.