It's the lack of what he has, is what he is desperately trying to hide. If released it will prove once and for all what a lying fuck he really is. He brandishes 10 billion as his total wealth depending on which video or report you see
Trump says he’s worth “in excess of $10 billion” and blames the “terrible”, “dishonest” media for underestimating his assets.
Donald Trump Net Worth | Money
If the tax returns came out the humiliation would be unbearable for him. If he was telling the truth there is absolutely no reason to withhold them!