That's not always the case,many caucasians' crimes have been covered extensively, but as for some situation that this has happened...
I can think of two reasons:
1-if the crime motivation has something to do with religion, then it is brought up. For example if a crazy person goes on a shooting spree because he felt like it, his mental state was the reason he committed the crime not his beliefs
2-the media can also be manipulating at times, sometimes they want to lead people's thoughts in a certain direction...