I have yet to hear any practical solutions to the following issues, but if you have them then I'm more than willing to listen.
1. Speed of deployment. Even China needs 5 years to build one on a site where there are already other reactors. In Europe it's 20 years. It's not due to lawsuits or anything like that, e.g. all that was resolved for Hinkley Point C (the site of existing reactors) and it is still taking 20 years.
2. Cost. Nuclear is eye-wateringly expensive. The UK, the world's 4th largest economy, can't really afford it, and had to get Chinese investment. For smaller nations it is even less viable.
3. Proliferation is a real concern.
4. Nations are understandably not keen on relying on foreign technology and expertise, and developing their own is expensive and risky (financially and in terms of safety).
5. Grids are moving away from "base load" suppliers to demand shaping and reacting to availability of renewables, and nuclear is not good at varying output to integrate with that. We have seen that already in California, demand drops to zero during the day due to the amount of solar installed. Any solution like adding storage can be applied to cheaper renewables.
6. Safety is still an issue, and so far claims that a reactor is completely safe and unable to fail catastrophically have proven to be, shall we say, "optimistic".
7. Fuel supply is a concern for many nations, as is disposal of spent fuel.
By the way, I know about thorium reactors (every prototype has had some kind of serious defect) and Small Modular Reactors (most of the downsides of full size reactors, worse fuel efficiency, and decades away from commercial mass production). If you want to suggest those as solutions, please address the issues I highlighted with them as well.