Simple answer = Heat
You want to have the lowest voltage possible and still have transistor conduction. Since power equal voltage times amperage, if your voltage is higher then you power will be higher and your heat dissipation will be higher. Consider the Haswell 4670K, it has a typical power consumption of 68.25W @ ~1.15V with an operating temperature around 37-41C. Now multiply the voltage by 10 (12V) and you would get a power consumption of around 682.5W
. Could you imagine the heatsink that you would need to cool this thing.