The term "races" in D&D is actually referring to something closer to species (and/or aliens). So when you ask for "subraces for humans," you're actually asking for humans to be split into different species. To continue an example, it'd be saying something like: "Nords are smarter than Bretons." This is a big no no, especially when you'd have to associate certain human groups (with their specific combination of skin color and other physical features) with these bonuses.
The differences in humans in D&D should entirely come from Backgrounds and/or the Variant Human free feat that is, importantly, not tied to any specific human society/culture/group.
Urban Dweller? Take the Criminal, Merchant, Noble, or Urchin Background.
Wilderness Dweller? Take the Far Traveler, Hermit, or Outlander Background.
