What was the main reason that the southern economy remained largely agricultural

The southern economy remained largely agricultural mainly due to agriculture's importance to the colonies as a trade export (especially cotton, tobacco, and other cash crops).

Supply and demand. Everyone needs food and textiles (cotton). The south just happened to be the only place that could really support agriculture on a massive scale. Since the south’s main resource was its good weather and soil, it was agrarian from the beginning. Once the culture and infrastructure for farming was in place, there was really no reason to NOT have an agriculturally based economy.

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