In A Nutshell: How the Media Works

From Tawana Brawley to Crown Heights to the Jena 6, to the Duke lacrosse team, the media will always latch on to a race-related story-or more often than not create one-to convince people that America is still an out-of-control racist cesspool.

With the George Zimmerman/Trayvon Martin story in particular, the liberal media has created a racial angle (and strife because of it) out of the story wherein there is none.
In a excerpt re-printed at Breitbart.com Ben Shapiro's book, Bullies: How the Left's Culture of Fear and Intimidation Silences America, explicitly lays this bare.