Different styles of play. When Chen was available, she started. She's the consummate point guard. She's the QB on the court. Heckel is in that mold. It isn't a question of stats, it's a question of managing the team operation.
But don't misunderstand. KK can and will do what Chen and Heckel cannot: she's a game-changer with her instant energy, furious defense, rushes to the basket. And, yes, she has improved greatly in all respects.
Look at it this way: KK is too important to be reduced to running the point. Her assets are unique, and Geno has employed her perfectly.