I suppose that's the norm for him, but there's no legitimate reason why he can't provide them. He doesn't want to. that's the sole reason. Which leads to the question why he doesn't.
Because he's under audit. There's no law stopping him from doing so, but, from a legal standpoint, it's a really bad idea.
Basically, if someone he does business with is disgruntled, they can pull up his tax information, find some line item they are somewhat familiar with and tell the IRS he's lying. Then the IRS is obligated to do more investigation into that area of his tax return. The claim could be completely bogus, made anonymously, but it will still hold up the audit while the IRS does it's due diligence.
Now, when you're talking about a controversial presidential candidate, you *really* don't want to release anything. Nutjobs would be coming out of the woodwork making claims about his tax returns. The IRS would be obligated to follow up on all of them.
Normally this isn't an issue, as there would be no reason to release your returns. Slightly different situation when you are running for president.
I can see valid arguments on both sides of the issue. Personally I don't care, but I could see how you could.