On the April 7th, a US airstike on Afghanistan killed 17 civilians, 12 of them children. But the newsrooms don't have live pictures to fill their 24 hour screensaver they call news coverage. If they did have footage, it would probably be too grusome to show anyway.
In contrast, the US beams us a saturation of images from Boston. There isn't really anything to see - police cars in the dark, an interview with a man who sdidn't see anything, a man in uniform giving updates. But all this pap is "live", unfolding, ongoing and fills the screens with a rotation of dofferent images.
The result is an entirely disproportionate amount of airtime to one story, and almost non of the other.
The Boston bomb was a big story to Americans. Less so for us. The British TV news media over did this story.