If I had to bet I would think that the government only retaining John Trump to ascertain what Tesla's research meant and to "translate" it into layman's terms that the government scientists could understand.

Tesla's research was so high concept that the government didn't even have a scientist capable of even comprehending it, much less making sense of it.

Remember, Tesla died in 1943. During WW2. The top government scientists were working on the Manhattan project at the time and were probably not allowed to work on anything else.

John Trump was probably sworn to secrecy and not allowed to divulge what he knew. At least not publicly. I think he talked about what he knew to close family and friends.