At 10Ghz, something moving at the speed of light can only go around 3 centimeters per clock cycle. Take out gate delays, and you wind up with something that just isn't practical.
The important things that changed were that smaller cores tend to leak more, which means that voltage had to be scaled down aggressively. And drive currents are now limited by velocity saturation.
The T2 processors from Sun had over 100 threads per chip. Its a shame that they didn't do better in the market place.
Our future CPUs should be tiny cubes, not flat chips.
Distance to each point in 3 dimensions is shorter.
Now turn them sideways, so they rest on the edges. Put the stack in a small ceramic container with copper bottom and top. Fill with a high efficiency thermal transfer fluid, and make sure the convection currents flow properly.
There you go, small matter of engineering.