Because the decision to charge Zimmerman is political.
Plain and simple. If you remember correctly, it was the Florida Governor who demanded Prosecutors to "cough *reinvestigate* cough" the case.
This is not a "justice" issue whatsoever. It's glaringly political.
So was the decision not to prosecute him when he was standing over a dead body with a smoking gun.
The decision there was, "We wouldn't want to get the gun nuts upset after they passed a goofy Stand Your Ground law." The cops wanted to charge the guy, the prosecutors said no.
I should point out the Florida Governor is a Republican.
My own view. Zimmerman is guilty in the strict moral sense. His actions led to the death of a kid who wasn't guilty of anything. Trayvon would be alive today if not for George Zimmerman. Now, maybe he can wiggle out under the law, but I hope not.