Because that spot on the SRB is a flex joint to allow some movement of the nozzle as the engine fires. The joint isn't there because of transportation or other reasons.
I'm no rocket scientist either, but happened to be reading in depth about SRBs in general and Challenger in particular, and my understanding is: the joint in question was a field joint, to be assembled in the field, as opposed to factory joints which were assembled in the factory. The construction of these joints were different (asbestos insulation vs O-rings). The differing design is due to transportation and logistics.