Some of the documents released on war.gov/UFO were classified in the past. For example, CIA-UAP-017 was classified Secret. The classification authority block is missing--possibly redacted--but CIA is involved in the clandestine collection of intelligence, which used to be classified under section 1.5(c) of EO 12958. When the classification authority block is present, it will usually contain a reference to EO 13526 (successor to EO 12958) or a security classification guide (which may or may not be public). Tracking down the authority will tell you why it was classified, at least in general terms.
Some of the documents do not appear to have been classified. FBI-UAP-D009 doesn't have the banner line or portion markings typical of a classified document. However, classification is not the only reason for withholding a document from public release. There are nine exemptions to the Freedom of Information Act and a number of laws that prohibit releasing certain information. Law enforcement agencies don't like releasing investigative material because it would compromise the investigation, or future investigations into the same subject.
Also, public release is never automatic. Even when a document is unclassified, everything has to be reviewed to ensure nothing is being put into the public domain that shouldn't be. The government has limited resources for these reviews, and they're usually busy on more important things than UAPs.
