For us non-aviators, why would this be classified in the first place ?
maybe the last message was
“Massive Jap fleet Chuuk lagoon, many fighter planes obser....
It's not about aviation but about her potentially being a spy, moving the U.S. involvement in the War in the Pacific prior to Pearl Harbor, and possibly earlier than 1937 when she disappeared.
I would suspect that the files have been classified because Earhart was possibly captured and tortured by the Japanese.
After WW2 was over keeping things like this secret was a practice for the US Government that needed Japan as an ally against the Russians and Chinese.