The magnetic fusion triple product* did in fact increase exponentially from 1970 to 2000, at about the same pace as Moore's Law. Then we reached the limit of the superconductors at the time, and could only move forward by building an enormous reactor which still isn't finished. Now we have better superconductors; fusion output in a tokamak reactor increases as the fourth power of magnetic field strength.
We also have lots of different designs to experiment with, and much better computers for running simulations. As the computers improve, fusion progress will speed up, if the funding is available.
* (The triple product of temperature, density, and confinement time is the critical fusion metric; for every fusion fuel there's a triple product above which you get net power.)