Well, if you're legitimately asking a question or asking people to question why they feel the way do, that's one thing. If you're encouraging people to consider perspective, I can understand that. But your question smacked of the rhetorical.
But there is also a degree to which it's very psychologically understandable to be more shocked by a deliberate act than by an accident. Accidents can be attributed to chance, fate, or god, depending on your perspective. They're no less tragic, but more... understandable in a way. But a deliberate act forces people to find a way to come to terms with something darker, a hidden corner of human nature. I'd posit that people who are distant from Boston but deeply moved by the events there are not so much mourning the dead as trying not to see something deeper about themselves.
As to your points about Iraq and Afghanistan, that may be media-related, but not, perhaps, in the way you're suggesting. In 2002, the images coming into your living room were gruesome, and shocking, and there was horror. More than a decade later, we're desensitized. Someone says, "Afgnanistan" and you unconsciously brace yourself. It numbs the shock. But when someone says, "In Boston today..." what follows comes as a total surprise.
Me, I was more shocked because I heard about Boston when my wife texted me saying, "I just heard two loud bangs and now there are cops everywhere. Did something just happen in Boston?" Then I couldn't reach her for 20 minutes. So yeah... this hit me harder.
So... if your point is that people should be more sensitive to the fact that suffering happens around the world daily, and we should do everything we can about *all* of it, then point well taken. But if your point is, "wake up sheeple, I'm smarter than you are" then you're both an ass and an asshole.