PSII produces oxygen, which can be used by rubisco for photorespiration. That would limit the efficiency of rubisco, and, thus, of the Calvin cycle. By not having PSII in the bundle sheath, photorespiration is reduced.
Work Step by Step
The key here is to pay attention to oxygen's role in photorespiration. One trick to make photosynthesis more efficient is to keep oxygen away from rubisco, which is the real subject of the question.