This is a matter of national security. The candidate's ties to foreign interests must be disclosed in order to fully inform voters before the presidential election.
Section 6103 of the Internal Revenue Code provides: “Upon written request by the President, signed by him personally, the [Treasury] Secretary shall furnish to the President, or to such employee or employees of the White House Office as the President may designate by name in such request, a return or return information with respect to any taxpayer named in such request.”
Another passage in the law says returns obtained that way should not be further disclosed “without personal written direction of the President,” suggesting that with such an instruction the returns could be made public by the government.