Several reasons why the ISG operating conditions are not met, and would prevent it from turning off the engine.
Common ones are engine coolant temp, temperature outside, climate control temp not reached.
Even when the ISG does turn off your engine, when it senses the battery charge is getting too low or the cabin temp is getting to far from the climate control set point, it will need to start the engine again prematurely.
I do agree though that the threshold for battery charge is set too high for ISG to function. Unless I drive for hours on the highway everyday, the battery hardly ever has enough juice to power ISG. Only when I charge the battery to full with a charger will it restore ISG function for a day or two.