Since he won't release his taxes, we get to continue to speculate why.
Today's guess: political contributions. A guy in Willard M. Romney's position doesn't give, he invests. Political contributions were given to secure favorable treatment from people or groups Romney needed - endorsements, attacks on opponents, or maybe just staying neutral when by logic the recipient ought to favor someone else over Willard.
That being the case, the pattern of donations over the years would reveal a Romney with no core, no clear ideology, just self-interest. And that means that there would be an embarrassment of riches for attacks on Romney, since almost every voter will be able to find something to dislike in the list of recipients.
Of course, he would also become accountable for the actions of the LDS church in the political arena. Which is another thing he intends to never talk about.