The release of tax returns has always been a political tradition. It is not required by law. If Trump wants to keep his finances private, that is his absolute right. We may not like it. We may be suspicious of it. We may not vote for him because of it. But it is just one of many considerations when we evaluate a Presidential candidate.
All throughout the Primaries (and in the General campaign), Trump has been totally about breaking with convention, breaking with traditions, breaking the rules of politics, breaking the rules of the media. He has been about anti-Political Correctness. He has been railing against politics as usual. He has been about upending Washington. He has been about disturbing the shit. (And everybody loved him for it!) So why should tax returns be any different?