For months, Trump said he would release his tax returns. Then he said he was getting them ready. Then he said he would release them once his audit was complete. See RedState for the details.
Now Trump says he won’t release them. Nothing is preventing him but himself.
As Jennifer Rubin points out, GOP delegates can apply some pressure. Trump may have opened himself up to a contested convention, delegates or not. Delegates should abstain unless Trump provides the information.
This is another promise broken and he hasn’t even gotten to the convention.