Probably because Preistap flipped and was given immunity. Remember that he retired only a year ago, quite a long time after a lot of the other FBI firings and retirements.
They had to keep him in the department to get all the info from him.
Remember this
Retired = full pension
Fired = No pension or partial pension depending on negotiations.
As if Sessions, Rosenstein and Wray were actually trying to get information from Preistap??
No way.
The only thing those guys "flipped", was the bird, to us.