Wouldn't a completely uniform building code in a country as vast as the U.S.A. impose some quite unreasonable conditions on everybody to cater for things which are only likely to be a problem in specific areas?
Does it make sense to demand that buildings in Oregon be built to the same tornado-resisting standards as in Mississippi? Or for buildings in Florida to be able to cope with the same snow loading as Minnesota?
It's not so relevant to the NEC, but the other buildings codes (IRC, IBC, IMC, etc) take this into account. There are tables for every county in the US requiring michigan to have thicker insulation in their walls than georgia. There are wind-load charts showing max 50-year 3-minute mile and fastest-minute speeds and pressure loads for all areas of the nation, with variable requirements depending on whether you're surrounded by trees and mountains or exposed on the coast. Same for seismic, termite, etc. In this way, they have a single common code that applies to the entire nation, yet is still tailored to specific requirements in specific locales.
If the beancounters determine romex is fine for most areas, but MC is required where housing density turns risk to a single structure into risk to an entire city- that should apply to everywhere in the nation that meets those criteria. Etc.
Maybe NEC needs to take an IBC approach and add a table for rats?
