The Boston Marathon was the lowest hanging fruit as far as ease of penetrating a sporting event. Whoever did this likely had limited resources, so took the opportunity to hit the easiest target.
It shouldn’t have been an easy target. DHS was all over the Fish Line area. Trash cans should not have been there. It could just as easily been an Occupier who wanted to punish evil corporate sponsor John Hancock Financial. The bombs also went off long after the Elite Runners from Africa passed the Finish Line. What Racist would let them get by safely?