Cops Convictions in Danziger Bridge (Katrina, New Orleans) murders overturned

Months ago this blog covered the Danziger Bridge murders (shortly after hurricane Katrina in New Orleans.  5 officers were convicted but their convictions were overturned because of prosecutorial misconduct. (The link also has good background on the cases.) Looks like the cops aren't the only ones who aren't playing by the rules.  Both police and prosecutorial misconduct are rampant in this country.  However , the problems never seem to get fixed.  Why do you think this is?