The answer according to James Comey was a "chickenshit" culture wherein people were afraid to take on cases that weren't a slam dunk. And it was difficult to make a case that bank executives were guilty thanks to the complex structure involved.
At least some of the issues in our justice system stem from not having the resources to handle every case fairly. Every criminal case should be fully investigated and tried in court, but the system would break under these conditions.
So you end up with a highly perverse system based on extortion, and heavily biased depending on socioeconomic status. Sometimes it works "too well", other times it doesn't work at all.
When you award promotions based on, say, the total number or total percentage of successful convictions, then the whole culture starts to optimize for that. Given the sheer number of cases that could be pursued, naturally the easiest-to-win cases will be selected, and more often than not those are small potatoes.