For a POTUS who has a long history of deceptiveness, selfishness and misrepresentations of fact, and who also refuses to divest of his personal business holdings while in the office of the President, disclosing his tax returns would be about the best possible way for him to at least provide the information the public needs to have so they can at least know whether his positions are apt to be personally profitable to him.
For me, it's a matter of scale. The man's a billionaire. If he advocates a position that happens to yield him a few hundred grand, I don't care so much. If it's a few million, well, I sort of care. If it's hundreds of millions, I care. If it turns out that his policies form a pattern of consistent gains great and small, well, I definitely care and I'd be inclined to think he's abusing the privilege of being POTUS.
I could take a somewhat less strident position were he to divest truly of his holdings, but he's not done that and shows no indication that he will. Moreover, his kids are on his staff and running the companies. That's just too cozy for us to also have no very detailed visibility to what's what.