Why didn’t Zimmerman stay in his truck?
Because he wanted to see where the suspicious person was going?
One could as easily ask, Why didn't Martin go home?
Both people had a legal right to be where there were, and do what they did until the physical altercation started.
He is , or was , the Neighborhood Watch Capt. and he was in communication with 911 reporting on Trevon’s suspicious activity.
From what I have been able to gather from maps of the neighborhood and the 911 recordings, Zimmerman lost visual contact when Martin went between two townhouse buildings. He left his truck, because there was no way to drive on a street to see between those buildings, and walked a very short distance, according to the maps, in order to see where Martin went. It was there that Zimmerman was surprised and assaulted by Martin, who had doubled back. There is at least one witness to that assault.
Zimmerman will be acquitted. He will undoubtedly be a very wealthy man too, as soon as he is through suing the various media outlets, the special prosecutor, the State of Fla., Martin’s family, Martin’s family’s lawyer, and everyone else that I have not included here.