Because it would have been obvious to anyone who actually looked at the tax return and instead of hiding it and then looking like a fool, he decided to get out in front of it. That's probably the right play, but it's hilarious in any event.
Also, too, Romney basically setting a floor for his own effective tax rate (at least in returns he has released publicly) is an implicit concession that the Buffet Rule to set a minimum acceptable income tax rate for very rich people, has merit.
Edit: And he didn't defer income. He just didn't take his full charitable deduction. Of course, if he looses the election he can file an amended return and take advantage of the full deduction.
or even if he does not lose the election
by the way, did he not say that anyone that pays one dollar more than he has to should not be president?