The one that died was wearing a suicide vest. The survivor is likely wearing one as well, plus they had a lot of explosives in their vehicle. They are trying to avoid providing soft targets for a likely suicide bomber. They have lightened up somewhat, e.g. allowing taxi service, while trying to prevent gatherings of people that would provide a convenient target. Contrast this with the cowboys-in-action that we saw in L.A. & San Bernardino Counties when the ex-cop went on a rampage. The police in the Boston area seem to be much better managed.