People have started investing in fusion. Tokamak Energy and a new company spun out of MIT's fusion program have both gotten tens of millions, for fairly mainstream designs. There are also companies using more speculative designs, the biggest being Tri Alpha, which started in 1999 I think and has about $500 million invested. There's also Helion, which got a modest investment from YCombinator.

One reason fission moves slowly in the U.S. is the NRC. A couple years ago I got to sit in a meeting between a bunch of U.S. reactor startups and a former head of the NRC. The reactor people's main complaint was that the NRC required detailed blueprints before it would even look at a design. It would take several hundred million dollars to get to that point, and then the NRC would give a flat yes or no. With a no you were done, and with a yes you still have nothing but a paper reactor. It's a very difficult environment for investors.