Because it's very hard to stop accidental or incidental information leaks. For an opinion to be rendered, generally they also attach the case facts which lead to that ruling. Heavy redaction means it becomes almost impossible to use that ruling as precedent but also runs a real risk of exposing state secrets. For example, which judge made the ruling? Well if you get that you can start pulling information about the days the judge was active and a short list of the people the judge was hearing cases for.

All it takes is 1 accidental redaction or someone who did something silly like using a black highlighter on a PDF and now where the nukes are stored is public information.

There's also simply information that can be gleaned just based on the location that the hearing is taking place. This came from x court in x district, well then it's likely a nuclear silo is roughly in that county or surrounding counties.

A federal court tried to conceal the identity of a judge who has been reprimanded.

Claude was able to work out who it was in less than 10min based on stringing together facts that remained and seemed irrelevant. Journalists uncovered the identity independently using different means. (The ability of Claude to do this was not revealed until the other journalists had gone public and confirmed the identity.)