What is this president hiding?
If you are using that as your logic, then you must ask that of every President before Obama. NO president had released his college transcripts. Someone got a hold of part of George Bush's transcripts and the New Yorker released that information - but that was not with his consent. Something similar happened to VP Al Gore during the 2000 campaign. And John Kerry released his transcripts AFTER the 2004 election.
Again, no presidential candidate has ever been required to release any of this information.
And you wrote that they all have:
that is a lie. To this day, George W. Bush has not released the rest of his transcripts. Bush 43 never did. Neither did Clinton, Reagan, Carter, Ford, Nixon, Johnson, Kennedy, Eisenhower, etc.. none of them did, probably because most people back then were decent enough to realize that this stuff is nobody's business.