The kind of logic cell used uses very little power to maintain itself in one state or the other, but it costs a little energy to flip between states. The faster you clock the CPU, the more often those flips happen. Heat can only flow out of the die so fast, so when you start hitting the limit on temperature, you necessarily can't clock any faster or you're going to burn the chip. All the power consumed by these bit flips turns to heat. So .. the power thing is primary, the speed is a secondary effect. If anything else, they might have better called it the 'thermal wall'
But why the 2005 is "power wall reached", isn't it GHz wall as well?