1) This is an American incident.
2) He was not arrest immediately after the killing
3) It has a race angle
Have not heard much about it in the UK, it is mainly an American story and covered on internet (like many other news stories).
Finally, to all those people saying it is about "white person kills a black person" - this is a part of the story, BUT not the whole reason for the attention it is being given, such as my 2) reason. Plus, white people have kill other black people on a number of occasions (not my main point as it can work either way), but they do not ALL make the news right.
This case is more than race, although race plays a significant role