My recollection is that it was the regulators that stood in the way of standardization by not allowing certification of a single design to be used to expedite the approval process. I remember proposals for standardized designs going back to the 90’s. The developers wanted to have a design certified, and then to allow a certified design to go through an expedited regulatory process that would not have to revisit the design but only site-specific matters.
But they were forced to recertify every aspect of construction with every new build, with changes to the design part of the regulatory mix if necessary, which blew apart the cost savings you could get from standardizing. In fact, you couldn’t really standardize much at all.