I just can't see how he is not guilty of anything...I guess the prosecutors just must have been all in on a murder charge.
Easy. Manslaughter makes no sense.
He did not "recklessly" kill Martin. It was very deliberate. You may be trying to say that he was reckless when he didn't listen to the dispatcher, but that's too removed from the actual killing to say that the killing itself was reckless. It wasn't, it was deliberate. His following of Martin may have been reckless, but his killing of Martin was not, it was deliberate. So it's murder (unless he has self-defense available to him). Manslaughter is for things like accidents, like DUI, where you don't intend for the victim to be harmed. Here, Zimmerman very clearly intended to shoot Martin. Not manslaughter.
If he was going to be convicted, murder was the charge that made sense. Manslaughter was a last-minute tack-on by the prosecution (he wasn't originally charged with it, and they only instructed the jury that they may find him guilty of it after both cases had rested) in an attempt to say "Damn! Our case for murder sucked so if you want to have pity on our ineptness, maybe just stick him with these even though it makes no sense?"
In both cases, however, self-defense is a defense to murder AND manslaughter.
So if you believe Zimmerman had the right to self-defense, he's not guilty of either crime.
If you don't believe Zimmerman had the right to self-defense, the murder charge is really the more appropriate one. Anyone who is an "aggressor" in a physical confrontation is not entitled to use self-defense.
In this case, the jury realized that there was zero evidence that Zimmerman was the aggressor (started the physical fight) and therefore he had the right to self-defense, and therefore he could not be guilty of either charge.
I know you want to say "BUT HE'S RESPONSIBLE FOR STARTING THE WHOLE THING BY FOLLOWING MARTIN" but for the millionth time, following someone is not a crime. Is Zimmerman morally to blame because he did something wrong by profiling Martin and following him? Maybe.
But that does not impute criminal liability here. So you can think he was wrong to follow Martin all you want, but unless you can prove that Zimmerman actually threw the first punch (or otherwise did something to be considered "the aggressor" of the physical confrontation that was MORE THAN following Martin), then there is no way to legally strip Zimmerman of his right to shoot someone who was very clearly pummeling him.