Seriously? OK, here we go...
1. Because the police never had probable cause to make an arrest, to say nothing of the prosecutor pushing the limits of the "I can indict a ham sandwich" theory.
2. Because the prosecutor charged Zimmerman with Murder 2, despite having a weak case. The only way Zimmerman gets any jail time is if he was offered a plea bargain that involved less time and expense than the trial.
3. Until the first punch was thrown, no crime was committed. Plenty of bad judgment, but no physical contact and no actual crime. The first punch was the first crime (assault). Not only did the prosecution fail to establish who threw the first punch, they knowingly went to trial without any hope of establishing this critical fact. To this day, nobody knows which person was the attacker and which was merely trying to defend himself. Meanwhile, the prosecution had to prove Murder 2 beyond a reasonable doubt, when they couldn't even prove who threw the first punch.
4. From the beginning, emotional people were anxious to convict Zimmerman of _something_. When they couldn't find enough evidence to convict him of a real crime, they took refuge in imaginary crimes or imaginary facts. It's not illegal to own a gun, to carry a gun, or to fire a gun at a person (in self defense). And it's quite possible to do something stupid but not illegal (such as following a person), and have the situation lead to self defense, even if a smart person could have made better choices to avoid conflict. If a person is dumb enough to start a fight with a stranger who just happens to be armed, there is no law that requires the victim to verify that the attacker has a firearm before using his own weapon in self defense. When the jury was limited to actual laws and the actual facts of the case, Zimmerman walked because the self defense theory was no less plausible than the stalker/murder theory. Neither theory was conclusively proven in court, but that's all the defense needs for acquittal.