Because the DA charged her with "Attempted Reckless Manslaughter", which the judge acquitted her of because "Attempted Reckless Manslaughter" is an almost incoherent crime. (Reckless Manslaughter doesn't require intent, Attempted [anything] does, this made the mens rea basically impossible to prove, and indeed it wasn't.) One would almost suspect that the prosecution felt the need to charge the cop with a serious crime but didn't want one that could possibly stick.
No jail time for Colorado police officer who locked woman in SUV struck by train (www.denverpost.com)