That "obvious reason" is a level of marksmanship and organizational competency that makes cell phone video coming out of Syria look like a documentary about an elite fighting force.
The previous time they tried to shoot him all they did was pepper a house in an upscale neighborhood. Then there was the friendly fire thing. They also peppered the boat pretty good. It's not like they didn't try. They tried to shoot him several times and they failed.
And that's just the tip of the stupidity iceberg when it comes to the Boston bombing. The worst part is they basically proclaimed victory on something that was a textbook failure at every step and the people of MA are so devoid of critical thinking skills that they mostly bought it.
Edit: replaced "brainwashed" with "devoid of critical thinking skills" because the former gives the government too much credit.