Everybody dreams up a dramatic reason. There is a legitimate reason for withholding some material, if that material implicitly reveals the classified technical capabilities of the sensors used, or infringes on classified or proprietary information. There is the question of what to do about information derived from analysis of artifacts. I really do not know, but I suspect it would depend on what that information is. When human wisdom cannot comprehend what is known, it also cannot comprehend the significance or effects of its revelation. Nondisclosure will often look to be the "Better Safe Than Sorry" approach.

The only thing to do, that I can see, is to appoint a sworn panel of auditors, cleared to examine this withheld material. And then provide their report to the President for disposition.

Otherwise (as some may suspect) this demand for disclosure may be a stalking horse for the outing of a lot of legitimately classified material that would have an adverse effect on national security (terrestrial, not interplanetary).