Zimmerman was charged with 2nd degree murder less than two months after the incident. I’m not sure what evidence was there when the charge was made nor what additional evidence if any was later developed. In support of the charges, the State filed an affidavit of probable cause, stating that Zimmerman profiled and confronted Martin and shot him to death while Martin was committing no crimes.
http://www.nbcmiami.com/news/George-Zimmerman-Not-a-Flight-Risk-Former-Attorney-146966625.html
I think it was because the witch and her flying monkeys found more exculpatory evidence to withhold.