Prosecutors have enormous power. Even investigations that don't result in any charges can ruin lives, ruin reputations, and drive their targets into bankruptcy. It has become an overtly political position -- in general, but particularly at the federal level. If a prosecutor wants to ruin your life, he or she can. Even if you've done nothing wrong, there isn't a whole lot you can do about it.
There are a number of factors that got us here, and it's worth looking at them in turn.