Answer :

Because they had slaves picking the cotton with a cotton gin and abolitionist were against slavery 

Abolitionism was the movement that began in the United States to end slavery. The members of this movement were known as the Abolitionists. The most important reason why the Abolitionists opposed the production of cotton in the South was that they wanted to put an end to the system of slavery. And slaves formed the main labor force in the production of cotton.