The Clintons' Tax Returns

This is the explanation:

"When she's the general-election nominee, she'll release the tax returns."

But that's not an explanation. I really can't see why this lack of transparency isn't a bigger deal. On what grounds is it legit to withhold critical information until the primaries are over? Especially when the vagaries of the Clintons' rapidly growing - and partly meshed - fortune is a serious campaign issue. Especially when a candidate has borrowed from herself to float her campaign. Shouldn't this be asked tonight? Shouldn't the press insist on getting the returns?