have you seen the photos? this wasn't a box of basic fireworks and a can of gas. the bed was loaded with a huge amount of camp fuel and large fireworks mortars.

considering that it was rented in Colorado, parked right in front of the trump hotel lobby with the trump hotel sign right above it, and exploded right after, the simplest explanation seems to be that it was intentional and that causing a scene was more important to them than killing.