The main thing is actually learning effects, the second thing is 'mass production'.
If you have teams that have build reactors before, they will be much better. Once you have people on their 3rd reactor, they go fast.
Its kind of like other massive infrastructure projects like bridges.
Basically nuclear reactors are large civil engineering projects, and you need experience to do it.
The second is that some parts, like the core reactor vessel are insanely complex high pressure and the production of these needs special facilities that basically only exist in like 3 places. They managed in the 70-80s to kind of 'mass' produce these but today its totally costume.
Another things is, France totally failed by themselves to build next generation plants. Its borderline crazy that we are still building PWR type reactors. All attempts at next generation reactors have been terrible.
I am really high on what's going on in Canada, real next generation reactors are FINALLY really seriously going threw real internationally recognized nuclear validation from a first class regulator.