Let's also shine a little light on the then Director of the FBI.
Robert Mueller.
There was some initial reluctance to investigate, because it started as a local Palm Beach case. Once the FBI investigation began, it was called Operation Leap Year. The gathered evidence became a 53 page indictment. The FBI isn’t the party that dropped the ball, here.
When deals are made by the US Attorney offices, that falls under the Attorney General’s purview not the FBI Director. When the deal was made, in 2007, it would have been either Gonzalez or Mukaskey.