I think half the problem with an electric stall turn comes from the lower than usual idle. The other half is more of a theory without much evidence to support it. My only evidence so far is my experience. My two electric planes run lower than usual rudder throw to accomplish things like knife edge, rolling circles, rolling loops, snaps, and spins, I suspect due to increased air flow over the tail from the huge prop. This increased airflow isn't present in the stall turn, so we don't have enough rudder throw in to rotate it properly.