There may not be a legal reason he couldn't release them now (I don't know if he claimed there was), but there is probably a political reason.

My guess:

There are probably some questionable things on the return. Perhaps they're perfectly legal, but there's no doubt that anything out of the ordinary is going to be scrutinized by the public, therefore, if he waits until the audit is complete, it gives him another leg to stand on in defending his legality (provided they don't find anything in the audit).

"Even the IRS found it to be perfectly legal."

It's more likely he's using it as an excuse. Trump was reportedly very critical of Romney releasing his returns (and the subsequent hit Romney took because of his decision to release them).

I was basing my guess on the fact that he's claimed that he will release them after the audit is complete.

Of course, there's always the possibility he's just saying that in hopes that we will have forgotten about it by then or that the election will have already taken place.

Straight up saying that he is never going to release them would probably be a big problem, which could explain the attempt to slide away from it, if the above theory is true.

His worry is probably that his returns would let people estimate his wealth and income. Trump's business is largely built around licensing his name, and its value comes from the perception that he is very wealthy. If it turns out that he is only worth a billion or less it could greatly reduce the value of his brand.