The CIA sources and methods of assassination need to be protected!
If there is such info, it will pertain to potential CIA attempts to kill Castro or otehr world leaders. There is no official government file that will state "the CIA killed JFK" or "LBJ ordered it", etc. Even if those things happened, the facts are not in some "secret file" waiting for the proper time to be released.
More likely they do not want the names of the assassins, and of course that LHO had nothing to do with the assassination. It was simply the same Klan that took out Trump, installed Johnson, Obama, Biden, Nixon et al. The election system in the US is what we thought of as the Soviet system growing up. It is managed, lower level officials are chosen, or used to be chosen by elections. Higher level offices are appointments by media/intelligence/military industrial complex and now big tech.