From what I've heard it's because Macbooks have really aggressive hibernation(I've experienced this on my MB Air personally). Basically they will copy everything from the ram to the SSD and shut down fully. Then when you wake up it copies everything from SSD back to RAM - but if you have 16GB of ram, then even with an SSD it will take nearly 30 seconds to copy everything back, and until then the machine is unresponsive. I think it would be better if they just told you they are doing it "please wait, waking up", instead of just giving you a blank screen.