Because the US covers a large area of land with contrasting climate regions.
10 land regions
4 main land regions
Nebraska has 49 land regions
Nogeroa has 10 land regions.
2 main land regions
three main land regions
Illinois has 3 land regions.
There are 4 main land regions.
There is three land regions in Costa Rica.
the great basin
Maine has 4 main land regions