I definitely think there is an obvious difference between this and the shootings in Baltimore or Chicago or elsewhere.
Trayvon Martin was walking home. He was not casing the neighborhood, running with a gang, looking for people to beat up, etc. He was returning to his father's home after going to buy snacks. Regardless of what happened between he and George Zimmerman, the sad, tragic fact of this case is that if Zimmerman had not been driving through his neighborhood at that time, Trayvon Martin would still be alive. THIS is the main reason why this case has received national news.
Unfortunately, tied in with that is the fact that the media, in large part, has decided that Trayvon Martin is dead because George Zimmerman is a racist white man and Trayvon Martin was a black boy. This was the story they ran with early on, showing photographs of George Zimmerman as scary and intimidating and Trayvon as a sweet-faced child. This was the narrative that was encouraged by celebrities like Spike Lee who encouraged vigilante justice by releasing Zimmerman's parents address on his Twitter feed or Jaimie Foxx who continues to wear a shirt of Trayvon as if George Zimmerman is guilty already and not entitled to a fair trial.
The case is receiving national news because 1) it is tragic and 2) because the media wanted to prosecute Zimmerman based on race from the first minute the case caught attention.