Because it's asilly argument. Giving up his birth certificate didn't get the birthers to shut up.
It's a zero sum game for Obama. He gives them up, they'll claim they're fakes. He doesn't, they'll claim he's hiding something. None of which has anything to do with his ability to be president.
Romney, on the other hand, has bet his whole wad on Bain while trying to deny what Bain actually did to real people.
It's a zero sum game for Romney as well.
Romney's tax returns will reveal nothing about Romney's leadership of Bain, or what deals he was involved in, or even what he made on individual deals.
Have you ever filed a tax return?
All that will be listed related to Bain Capital is his total cash compensation on one line. Period.
What this is is a fishing expedition, nothing more.
Just like Obama's college transcripts.
So, if one side wants to go fishing, the quid pro quo is the other side should get to fish too...nothing is more fair than that.