Nuclear fusion power was supposed to be a dream come true. As soon as we discovered that you could smash little atoms together to make bigger atoms and release a small amount of energy in the process, scientists around the world realized the implications of this new bit of physics knowledge. Some wanted to turn it into weapons, but others wanted to develop it into a clean, efficient, inexhaustible supply of electrical energy.
But it turns out that fusion power is … hard. Really hard. Really complicated. Full of unexpected pitfalls and traps. We've been trying to build fusion generators for three-quarters of a century, and we've made a lot of progress — enormous, groundbreaking, horizon-expanding progress. But we're not there yet. Fusion power has been one of those things that's been "only 20 years away" for about 50 years now.
The primary challenge is that while it's relatively straightforward to make fusion happen — we did it all the time with thermonuclear weapons — it's much more difficult to make the reaction slow and controlled while extracting useful energy from it.
Not soon enough.