Kimberly M. Foxx is currently serving as the State's Attorney for Cook County, Illinois and was apparently the one who decided. In the US, prosecutors like her don't have to explain why they choose to drop charges. State Attorneys have almost no responsibility to explain their actions and can even let murderers go free when a solid case is against them.
It's a massive flaw in the US justice system, and a cause of corruption. The defendant's lawyers can literally donate money to the State Attorney's campaign funds, the Attorney can drop the charges against their client and it's legal.
