In at least one year in the past five, Romney likely realized enormous capital losses, on paper, in order to cash out an enormous amount of capital gains. In economic downturns, large shocks to net wealth can be leveraged to permit owners to 'unstick' tax-unfavorable positions and move wealth to tax-favored vehicles without ever taking the capital gains. This is a relatively routine sort of transaction for people with large potential tax liabilities, but which results in a sort of shocking-looking tax return.
From a tax accounting perspective, there are thousands of people whose tax returns look much like Romney's. From a political perspective, any factoid which requires one sentence to state and ten sentences to rebut is a winner. The best possible outcome for Romney releasing his tax returns is to provide Obama with a facile attack line which requires an elaborate explanation of tax law to rebut.
That won't happen, so Romney won't release.