Why do basement walls need less insulation than crawlspace walls?
The earth is a good insulator itself relative to cold air that may circulate outside a crawl space.  The temp of the ground even few feet down is much warmer (or, in the summer, colder) and more stable than the outside air.  If the air outside is in the 20s, the soil even a few feet down is much warmer (in climes as far south as yours). Ground source heat pumps take advantage of this fact.