I am, and have been for over 30 years, a retired Special Agent (SA) of the FBI. When I first entered on duty, there were no non-white SAs that I was aware of, nor were there any female agents, and few Hispanics or other minorities. All of this changed during my career, but obviously not to the extent where the SA population reflected the national population.
I think that one of the main reasons may be that (as the FBI's current recruitment drive says) it is a way of life -- a calling. It is my observation that black, and other minority, people who may be considering an FBI SA career are the same people who can command big bucks in the business world, and the FBI cannot compete on salaries. No SA ever got rich on his salary while an SA.
So, at least some of the perceived fault (and I feel most) lies elsewhere than at the feet of the Bureau.