Beyond the other answers you've received that mention the paper trail, district attorneys are elected officials and police officers aren't. The citizens of a ci

Beyond the other answers you've received that mention the paper trail, district attorneys are elected officials and police officers aren't. The citizens of a city choose the district attorney as a proxy to make decisions for them. They don't have that input when it comes to choosing police officers. The competitiveness of local elections and the level to which voters are informed aside, that is an important difference.