For the edification of the people, this should be an established practice. In this particular instance, I suspect the reason the president has not done so is because he is either receiving egregious tax breaks he'd like to keep hidden from hard-working, tax-paying Americans, OR he is establishing (as evidenced) that the unspoken rules and standards of former presidents and future presidents do not apply to him. Releasing the tax documents would have alienated many of Trump's supporters, but I firmly believe knowledge is power.