Prosecutors should be focused on justice. They should not evaluate themselves, or be evaluated on, the narrow metric of conviction rates. Sometimes doing their job properly should mean being allowed to change their minds, to admit that maybe they're prosecuting the wrong person, or to conceive that the interests of justice are best served by a lighter or suspended sentence. And this is how it works in most of Europe; magistrates are entirely independent, and not beholden to political pressures or to the court of public opinion.
Every case a prosecutor takes and fails to convict is an instance in which the state inflicted harm on a citizen that was later proven to have been unjustified.
Sometimes that's a risk worth taking, but not often. Acquittals are a bad thing.
