You can't be serious. Someone really lacks imagination, I see. Off the top of my head, I can think of several highly plausible reasons:
1) Bad grades.
2) Disciplinary problems that could embarass him.
3) Evidence he was a beneficiary of affirmative action.
4) By keeping them sealed, he encourages idiot conspiracy theorists like you, who then make the entire opposition look stupid.
Wow, is that what you teach at the University of Washington, or did you received the same talking points we heard all over the spectrum yesterday???
With all due respect you sound exactly like your colleague professor Gates. Maybe it's time for you to have a beer???