I think those would be public domain, or at least subject to FOIA. Having some of them not be posted on a web site probably isn't 'making them disappear'. I think, but don't know, there probably isn't a law or reg that they all be available immediately and online.
Also, the NTSB performance varies from exceptional to, well, just ignorant and/or misinformed. Some of their conclusions are the result of great science, superb insight, great forensics. And some have been just sloppy dumb. Maybe the "B" team handles the web posting content. And they are a Fed gov't entity; high performance not requried, expected, or likely.