There are several factors, but the single biggest reason for nuclear's ever increasing cost is due to construction productivity.

All other forms of productivity have gone up drastically. However construction productivity remains stuck at a constant. And as other areas are more productive, we need to still pay those in construction competitive wages or else they would switch to more productive jobs with higher wages. (Let's just elide the disconnect between wages rising fully with productivity increases, but they do rise some!)

Despite being far more wealthy today than centuries ago, we don't build cathedrals with super intricate stonework, because labor is so much more expensive.

I fear that nuclear is like the gothic cathedral: something that was far easier to do when labor costs were low, but at wealth increases it becomes far more difficult to make economic sense.

Baumol's Cost Disease

https://en.wikipedia.org/wiki/Baumol_effect