The only answer that comes to mind is that Hillary is involved and deeply and she has gone to ground to protect herself and her putative future (Wow, look at those critical missions to Peru and Australia that are more important than testifying before Congress). Susan Rice was the only dependable liar available to give the Obama reelection team cover.
It worked, didn't it?
It is a delay of game strategy just like Holder. That dude should have been gone long ago. They just lie, or say nothing, or say you will know when the investigation is complete. All the while, they are stonewalling the investigation! We then get frustrated and give up. And all goes on just as they wanted it to. We are in deep doo.
I've thought since this happened that, since the truth about Benghazi might have seriously damaged Obama's reelection chances, that Hillary knew what actually happened and had no intention of going out and lying to cover Obama's skinny rear end. I expect she told him where to shove it so he had to put Rice out there to lie for him.
Without getting into all the weapons running possibilities, simply the requests for additional security that were refused, and the refusal to send any help while the consulate was under attack, that was bad enough by itself to do serous damage to Obama. It had to be covered up just before the election and Hillary told him to shove it. And she probably got some advice from Bill on what to do, or not do.