You want to keep intelligence separate from policy.
Another factor is the decision made in 1976 to sharply divide the FBI and the foreign intelligence agencies. The FBI would collect within the United States the foreign intelligence agencies would collect overseas.
The standard rumor at the time was that Rumsfeld as chief of staff had persuaded President Ford to appoint George H.W. Bush as director of Central Intelligence assuming that that got rid of a potential competitor for the presidency.