That is a slightly vague question that can be due to many reasons. One of those reasons is because the north became more industrialized rather than dependant on slave labor. There were a lot of workers in the north that needed jobs that couldn't let slavery take over. Many New Englanders believed slavery was morally wrong. The Potato Famine caused many immigrants to come to the United states and even though they were looked down upon, they still needed jobs too, and they very well couldn't compete with free slave labor in the south. This accounts for the Irish immigrants. There was something that happened in Germany that caused a lot of them to come over. They released all of their prisoners in the United States with the promise that they would never return to the East. There are many more reasons where that came from, be more specific next time.