The startup people said their biggest problem was the NRC's regulatory model. All the current regulations are written for conventional reactors, with mandates that don't make sense for other reactor types.
If you want to build something else, you can, but you have to spend a couple hundred million dollars on up-front design. Once you have that, you submit it to the NRC, which only then decides whether you can move forward and actually run some experiments.
That's a pretty difficult environment for investors. The startup people said if we at least had a staged process, giving them some kind of hint about whether the NRC might approve the project before spending all that money, then they'd have a much easier time getting investment.
Because of all this, reactor startups have been going to other countries. Terrapower is building a demo in China, Thorcon in Indonesia. Terrestrial Energy's in Canada and has good things to say about regulators there. Congress is starting to move towards reforming the system, but it's slow going.
Some of these designs have dramatic safety and proliferation advantages. Some other interesting startups are Transatomic and Moltex.