You will see the firefly population decreases significantly as you go up north or far south, near the poles where the temperature is lower.
Lightning bugs are more common in parts of the world that receive more rainfall and have more humid environments.
North Carolina as a whole has relatively warm temperatures suitable for lightning bugs.
Fireflies thrive in moist environments, so it makes sense that Boone (an area with relatively high rainfall) is famed to be the "Firefly Capital of the World." Additionally, the mountain region of NC has more forests, fields, streams, rivers, and lakes.