I'd honestly say it's less a matter of cultural diversity and more a matter of administrative ineptitude, MZ. Fact of the matter is that while we, conceptually, have the software (or means to create it, anyway) and methodology that would be needed to keep things running without sectioning the country into subregions, much of our governmental systems are still partially or fully analog, and much of our administrative body is either uncomfortable with, incapable, or unwilling to get(/getting) up and do(ing) a proper digitalization of, well, everything. Basically, while we are in the information age, much of our logistics and governmental/administrative systems are still lagging a generation or two behind*. I'd wager it's going to take at least one more guard change (older, entrenched generation dying/retiring off) before we'll have an administrative body that would actually be capable of setting up and operating within something like what you're proposing, and even then it will probably be another generation or two before stuff really starts getting closer to it.
Beyond that, from a logistics angle it's usually a really good idea to break a large area into smaller ones, especially when a physical presence is necessarily involved (information is near instant, eyes on ground can take much longer, etc., so forth, so on.). States as-is aren't exactly an ideal sectioning, but they work well enough, and given that existent systems are already set up to work with them rearranging things is likely going to be more trouble than it's worth for quite a long time past the point we're capable of better optimizing things.
tl;dr version: Old people are old, and beyond that states are actually a layer of useful bureaucracy, at the moment.
*You see this what seems like almost every damn time something major happens. Game releases (Server going down under strain, despite the people behind the curtain in a position that they should seriously be expecting the level of strain involved), major new digital integration (The varying snafus the ACA's electronic rollout has been accompanied by is a grade A example), etc., so forth, so on.
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