- Nuclear power plants are very complex. They are more "engineered" than "constructed", with hundreds of kilometers of tubes and wires. Pouring the concrete is the easy part.
- Nuclear power plants deal with an incredibly dangerous environment: high pressures, high temperatures, radioactivity, hazardous chemicals. Everything has to be tested, re-tested, and certified. Many parts will be near impossible to replace once it enters production, and the plant is supposed to be operational for decades.
- Nuclear power plants are safety-critical. Contrary to many other structures of similar complexity, things can get way worse than a big explosion. They have the potential to contaminate the site (or even the surrounding country) for decades or centuries. Failure is simply not acceptable.
- Safety regulations have been getting stricter over the years, because supposedly "100% safe" plants keep having accidents and the population is not very happy about that.
The existing stock of nuclear power plants is an offshoot of several military programs. The most common design is pretty much a submarine power plant on steroids, which turned out to be less than ideal. Things like "safety" and "profit" were almost seen as a suggestion more so than a requirement.
Newly-built power plants are required to incorporate over 50 years of innovation, but it turns out almost nobody has the skills to actually build them. They have simply gotten too complex to construct!