Stall speed: flap deployment generally increases the stall margin - turning, decreases it. Stall concerns would seem to be an unlikely training point, unless flap retraction was the concern. Meanwhile, flap asymmetry recognition is certainly reasonable in training.
The most obvious reason, perhaps: Deploying the flaps adds an extra layer to pitch trim for students. For most aircraft in standard approach configurations, the two actions cause changes in wing load and configuration where pitch requirements are opposite upon turn entry, and additive at rollout.