One reason is the close relationship between police and the judiciary. Both work closely together and need to preserve that relationship for various career and practical reasons. In theory a branch is designed to act as a check against another branch, in reality, this is not the case in our justice system.
Another reason is US voters. We vote "hard on crime" people into office and then are surprised when they give Police complete immunity. If we want justice, we've elected the wrong people to give it to us.
It's also not clear if any of this is politically viable. The US is politically farther right compared to Europe and some other countries. Is a more liberal "European style" justice system palatable to US voters? For some, definitely, but overall, I'm not sure if some parts of the US are culturally ready for some of the reforms discussed. There's a lot of "eye for an eye" syndrome here.
I don't have any answers to this, just adding some context.