We know why Romeny is hiding his tax returns.
Because we would discover that Romeny legally and properly didn't have to pay ANY TAXES during that period.
And the GOP is fearful that even loyal Republics might get a tad upset when they discover that Billionaires like Romney can game the IRS so easily.
Romney is not even close to being a billionaire and it is the job of the IRS to audit tax returns that are suspected of violating tax law. The IRS, controlled by Obama, would have discovered any 'gaming' done by Romney if there had been any, and I have read that they can go back seven years.