Along with failing to develop things like industry, and shipping, and infrastructure, the south had also failed to develop anything like a financial sector. What that meant practically was that the southern planters sold very little cotton directly to Europe. Instead it went to New York brokers who had fronted them the money they needed. Those brokers sold to anyone who met their price. Northern mills could buy as much as they needed at the going rate, and didn't have to pay for the transatlantic shipping.
Didn’t the North also put some taxes / restrictions on cotton going to Europe? It is my understanding they did.