IRS is bound to confidentiality. People need to know for...

IRS is bound to confidentiality. People need to know for instance whether he is indebted to overseas interests that could create a conflict in relation to economic or security considerations. Does Trump and companies owe a lot of money to Russia or China?
If they do the voters could well wonder why his over the top praise for the Russian President. People need to know what conflicts of interest could arise etc.

He stands alone on this the only candidate in modern times to not make public his returns. That in itself raises ample suspicion as to what could be found.