The law favors the officers
The reason is clear. Officers used "discretion" when dealing with the public. Police brutality happens when, the officer abuses that discretion. The problem is that, to prove it, you have to find another officer who would say that "under the circumstances, he/she would not have acted the way the officer did." Very difficult to do.
The officers have qualify immunity
The doctrine of Harold v. Fitzgerald gave officers "qualify immunity" and many Districts view this as an obsticle, that Plaintiff must surparss, before discovery can be conducted.
Too many frivolous claims makes it difficult for real claims to be handle
Every criminal that is placed on hand cuffs, will almost inevitably complaint that "the officers used excessive force." This creates a back log of cases that would otherwise settle, when defendants do violate innocent people's civil rights.