Because your description of what happened is not, in fact, what happened. Zimmerman did not physically block Martin.Where I live, if some random guy I've never seen follows me down the street and blocks the sidewalk on me in the middle of the night, prevents me from walking away, I can hit him with everything I've got in self defense. What Zimmerman did is assault, in Minnesota.
That was the part that has been puzzling me: why is everyone on TV talking about Zimmerman's self defense, after he assaulted someone and then shot them as they fought back?