I would imagine that it was because the British wanted to pay with goods and not money, where possible. Britain, having been the First Nation to industrialise, was importing raw materials and exporting finished goods (indeed, the British Empire provided a source of cheap raw materials and a captive market for the goods made from them). But any people’s having to pay a heavy tariff on imported manufactured products would likely not be able to buy as many and would actually buy fewer (is this not what 45 is trying to achieve now with China, with US cars built outside the USA?) So, if the south cannot buy as many British widgets, the British would (and did) get their cotton from elsewhere. India, I think.