Why did the north want to keep slaves?

The Northern states, for the most part had outlawed slaves. Some states abolished slavery even before the US Constitution was ratified. There was a segment of people in the North that directly and indirectly benefited from the slavery in the South. Northern owned textile mills needed cheap cotton, and slaves working on Southern cotton plantations provided cheap labor and thus kept the cost of cotton lower than if free and paid farmhands worked in the cotton fields. Some Northern people, in fact many by today's standards were racists. If slavery was abolished then some former slaves might migrate to the North. Not only did racist people oppose that, but in certain labor unions they believed that any new free Black workers would drive down the pay rates of factory workers.