Safety standards were set higher and higher because accidents kept happening, resulting in some well-known extremely large scale disasters and numerous minor ones. As with every industry, the rules were written with blood.
When you are building a power plant which has the capability of making a significant portion of your country permanently incompatible with human life, you generally want to be really sure you aren't going to have an oopsie.
When you are building a power plant which has the capability of making a significant portion of your country permanently incompatible with human life, you generally want to be really sure you aren't going to have an oopsie.