The reason is plain:
“Trump said that all the JFK files will be released, but the truth is that thousands of JFK files are still secret. The clear intent of Congress was to have these released last October and now we’re talking about 2021,” Morley said. “The point is that the CIA wants to keep this secret forever. It’s a very clear statement of intent.”
Source: Why did Trump delay full release of some JFK assassination files until 2021? – The Washington Post