Getting all of an explosive charge to detonate (rather than simply oxidize or be distributed by an existing blast) is already a challenge in designed explosives. This was a bomb, yes, but a happenstance one spread nonuniformly over a large region of warehouse. It's not unlikely that a considerable portion of the ammonium nitrate was blown away by the first-to-detonate material.

Speculation, very much not my area. Informed by earlier reading however.