Many readers are offering theories as to why Romney is still holding out. One writes:

Maybe Mitt is reluctant to show how well his portfolio did since January 20, 2009.

Another:

Maybe Mitt's being cagey on purpose.  Maybe there's nothing that revelatory in his returns, aside from a few more "geez he's really frickin' rich" items.  And then the whole festering mess feeds convenienty into the narrative of "Obama will do and say anything – ANYTHING – to try to discredit me."

Another:

You quote Kornacki saying, "Maybe the calendar offers a clue about Romney’s thinking. In just over a week, the London Olympics will kick off. If you’re going to release more embarrassing tax returns, maybe that would be the ideal time?" If I were Mitt, I would release my returns during the Olympics in the hope that they will be overshadowed, not by the Games themselves, but by the embarrassing fiasco of the Games' security contractor, G4S, totally botching the job and being woefully short of security just before the event. This has already been well-covered by the media (e.g. here), and I wonder if Mitt isn't sort of hoping this continues to be an embarrassment for London so he can compare these Games favorably with Salt Lake City 2002. He couldn't have timed this in advance, but if he's smart this could provide a useful, and distracting, contrast.

Another:

Maybe he's not releasing his returns because he did not tithe enough in previous years.

Remember, the Mormon church requires your to contribute 10% of your income to the church. Without it, you are not allowed to enter the temple buildings and perform certain ceremonies. They call it temple endowment. The only way the church verifies the 10% claim is via self-declaration. Once a year, you go to your bishop, who asks if you did indeed tithe enough. If you say yes, you are done.  This meeting is called "tithe settlement meeting". The idea is that you will not dare to lie to the bishop, who represents the church itself. Apparently, you do not have to bring any proof if you do not want to.

Now, in the one return that he has released, you will notice that Mitt has contributed near-exactly 10% to the church. What if he had fallen a tad short in some of the previous years? Mitt cares a lot about his church, and his standing in it. That standing would take a massive hit if it turns out that he was, er, "cheating" – even a little bit.

Another:

I was thinking lately how Romney could turn things around on the subject of his taxes:

* Call for a news conference.

* Announce that he is releasing 10 years of tax records.

* Explain to voters that the tax system is broken.  Point to his Swiss bank accounts and tax shelters in Bermuda and the Cayman Islands and his paying a lower tax rate than most working Americans as proof that the system is broken.

* Admit that, yes, he has gamed the system.  And if possible, do his best to claim that he broke no laws.

* Explain that because of his experience, he is uniquely qualified to clean up the tax system.  Explain in detail viable proposals of how he intends to make the tax system fair for everyone.

I don't know if it would work or not, but it possibly could be a game-changer.  And while there would still be people rummaging through his records to find delicious details, his already admitting to gaming the system might lessen the impact of those stories.

Of course there remains a problem if there is ample proof that he broke the law.  And there's also the question of whether he actually wants to repair the tax system.  And then there's the problem of Romney's lack of political courage to attempt to pull it off.

Earlier coverage of the tax returns here, here, here, here and here.