The problem is if that electricity is being used for heat, the draw of the 2 hour on periods will increase to make up for the time off. Perhaps even resulting in more total usage, because when power is on, people will set the thermostat to 80 instead of 65, anticipating the power going off and it getting colder. When really you need everyone's expectations of comfort to change to 40 degrees (half the delta T).
The only way to win and supply some power to all houses is to keep the duty cycle so low that the amount of resistive heating equipment becomes the limiting factor (1 hour every 6 hours?). I don't know if they could have managed this, but it's certainly more complicated to figure out than simply lowering the period of the rolling blackouts.