He was not acquitted.

The charges were dropped once the massive pile of police incompetence met the shitstorm of public scrutiny.

First the charges were dismissed without prejudice so the prosecutor could dig and see if he could find some way to make it Walker’s fault that cops killed his girlfriend. Then, when he couldn’t (because of the aforementioned appalling incompetence), and public scrutiny didn’t decrease to a point where he could quietly pressure Walker into a cell anyway, they were dismissed with prejudice.

It is important to not make shit up about this. If the public scrutiny hadn’t been as intense, it is entirely possible that they would have dragged him to trial and pressured him into a plea bargain. He was lucky that the public managed to continue giving a shit for more than their usual 30-second attention span.