Star Citizen's website published an update blurb that details the reasons behind the delay.
It's also worth reading if you're a casual SC follower and weren't aware of all the stuff that 0.9 will bring to backers. There's the new Murray Cup racing map and game mode, a co-op Vanduul Swarm mode, and two new flyable ships (the M50 and the 350R). The update also features Star Citizen's first stab at private match functionality and a few other bells and whistles.
Cloud Imperium says that it "will be working around the globe through the holiday weekend to deliver on the promise of V0.9."