Good question. While I have no personal knowledge of the particulars in the redactions of this case file I can suggest that the bulk of the redactions are comprised of personnel data and speculative cya...game playing what-ifs...
There would be no smoking gun in the redactions...just privacy issues and embarassing speculation and PR snafus. That said, there certainly was a crash in Roswell that date and various agencies of the government have consistently lied about it. When the government goes to that much trouble to pull the shades on something it means only one thing...whatever happened started out routine and the further up the chain it went some wiseass...decided to make a mess of the facts...and J. Edgar always hated to look stupid...
While I don't believe it was an "alien" craft, I refuse to be so arrogant as to assume we are alone in the universe...let alone smart enough to escape our own gravity without a lot of noise and smoke.