Two points.
Could Mr. Trump have released his tax returns under audit? Yes.
Would any tax attorney worth his fee have allowed him to do so? An emphatic, NO.
Secondly. Every large real estate developer operates under similar tax structure and tax incentives as Mr. Trump and may also show many years in which they have no tax liability. That's the way politicians set up our tax system – carrot and stick, reward and punishment for financial behaviors.
NeverTrump and the democrat media should get a grip – but, of course, as we've seen now for 16 months, for political and emotional reasons, they can't.