Prob because of heat issues, from the cards being so close together. In an Aurora SLI cards are right next to each other, they are actually touching each other. And the Aurora doesn't have good air flow already, 2 GTX 780s will heat up the inside of the case like crazy.
Until they do something about the Air Flow in that case, I don't think they will add 780 SLI to the options.
You can install 780s SLI your self though. Just buy them and install them. I would liquid cool them though, if not, you will prob have issues with Temps.
