The reason is because the cotton gin produced so much more cotton, the slave owners wanted more production with the gin which meant more slaves are needed to operate the gins.
Without the cotton gin, slaves had to hand-pick the seeds from cotton one-by-one, and it was a lengthy process. But with the cotton gin, they were able to produce the cotton a lot faster.
This caused slave-owners to get more slaves since the cotton gin could produce so quickly with great effort and result.
