I'm not privy to the internal motivations of the President (or anyone else for that matter), though I can think of at least one reason besides the two that you've listed: doing so earlier would have made the issue that much more of a political football, principles aside (which is one reason why pardons and commutations are left for the end). I fight against being cynical as a rule. I'm not going to go out of my way to argue that this decision was made purely on principle. That said, it doesn't need to be purely craven posturing either.