Well it’s no ones business, first of all. Not a requirement to run for office. He was elected without publishing them. And the federal financial disclosures each federal official makes are far more detailed and contain far more information than a tax return as far as showing ones wealth goes.
He knows most of America, and especially the left, is completely ignorant as to all things economic and tax. However, if his return includes an 8938, foreign tax credits, etc. like hundreds of thousands of other taxpayers have, the left will spin that as something nefarious to the ignorant masses.
The frothing at the mouth loons on the left want the tax returns. That’s reason enough to fight tooth and nail not to release them.