I think the reason they opted for this agent is due to the rapid onset of its effects. A more conventional agent, like your run of the mill 'sleep gas' would induce sleep, but the speed of onset would be slow enough to make some terrorists become aware of what was going on and detonate the devices. However the use of a binary nerve gas like sterase inhibitor is something that will conk everyone very fast. Meaning that by the time a terrorist discovers something is amiss he no longer has the ability to channel nerve impulses from his mind to his muscles. He wants to blow up the theater but he is basically petrified.
That is why ...i think ....they used the agent. Using a less potent substance would have led to at least over a dozen of the terrorists to notice there fellow comrades are falling asleep, and they would have rushed to detonate the explosives lining the theater. Carnage would have ensued and scores would have died.
However add a binary agent and suddenly you have everyone in the theater getting turned off like a switch almost simultaneously.
As for the sleeping part i think it was the media seeing limp people scattered around. The most orthodox thought is 'a sleeping gas' when it was probably a nerve agent immobilizing them.