Trumps tax returns are hella complicated. Have you seen them?
I tend to agree that the D's wouldn't understand them and he likely has many business entities that are rolled into a holding company of sorts. If that is true, it would take an army of accountants and tax people to pick them apart.
Its a rabbit hole for them to chase headlines where all he'd be doing is "see page 455 for the counter" Its a no win situation for him.