That one, though, is because we are running into physical limits: if we want to build things out of atoms, we can't make features that are half an atom thick. Even above that scale, physical effects that used to be ignorable, like quantum tunneling, no longer are.
From the late 70s through about 2005, scaling semiconductor generations was easy. MOSFET scaling followed rules formulated by Dennard, which provided a fairly easy method of scaling semiconductor designs from one generation to the next, keeping power density roughly constant and continually improving performance. The problem is that by around 2005, if you did it that way, your gates were no longer switches, they were dimmers, and leakage power started to dominate, and that meant that chip architectures had to change radically to keep on scaling.
So, we can no longer just scale designs from one generation to the next, we have to come up with completely new approaches. That's much harder.
https://en.wikipedia.org/wiki/Dennard_scaling