Because of the President constantly attacking and attempting to sabotage the agency, including firing its head for transparently political reasons.
If I were in law enforcement, I'd certainly think twice about taking a position there. I'd be asking myself "what if I get drawn into Trump drama? Would that be good for my career?"
I don't mean to overstate things—I'm sure the FBI is mostly composed of talented professionals. But I'd be amazed if the recent drama had no effect on their ability to recruit.
