And everyone with half a brain knows that in a potentially controversial case most officials understand it's best to take the cowardly way out (an arrest). Which suggests that they knew they would end up with an expensive prosecution that amounted to nothing, and thought the evidence was already so strong on the side of self defense they didn't have to take the cowardly way out.
State law awards damages to wrongly charged defendants, in self-defense cases. So, not only would there be the cost of prosecution (essentially zero for the state, as the prosecutors and judges are on payroll anyway, the incremental cost of trying a case is small), there would be damages to Zimmerman.
Then there is the risk of looking like Nifong.