He won't release them because there's campaign-ending sh*t there.
See, Romney supporters? This is the sh*t we need to kill by releasing these motherf'ing returns, and sooner rather than later.
Romney would not have released the dossier to McCain's camp if the above claim were true. We all know that. It is, one, just common sense (If Romney had anything in his background that was "campaign-ending," he never would have ran for president to begin with). The McCain camp did not ask for 20-something years of returns. Two, if the returns had "campaign-ending sh*t" we know somebody in the McCain camp would have leaked the dossier last year to kill Romney's chances in 2012, and pave the way for somebody else they work for.
Us politicos understand what I am saying, but the public at large is duped by Reid, et al.'s nonsense. All the Democrats have in 2012 is "attack and blame." So much for "hope and change."
You're neglecting the possibility that Romney accepted the IRS amnesty for a tax-evading Swiss-bank account in 2009. If he did so, the relevant information would not have been in returns vetted by the McCain campaign.
Of course McCain didn't pick him. One can only speculate about what is in his returns that he won't release to the American people. Often a mortgage will require more years of tax returns than Romney thinks that the Presidency requires.
One thing we do know for sure though, is that Ryan wants to eliminate capital gains taxes so that Romney pays 0%.