The lack of the crater doesn't bother me yet. First, it could have been placed on the bench, not worn as a vest. More evidence will clarify the matter. What bothers me about my theory is that there were apparently no nails or shrapnel attached, which one would expect from a homicide bombing. This story is weird no matter how you look at it.
If it detonated on a thick reinforced concrete road, let's say (not a bad idea for heavyweight vehicles such as might visit a stadium), there might not be much of a crater.