In 1790, the state in America with the highest population by far was the southern State of Virginia. However, by 1860, Virginia was surpassed in population by Illinois, Indiana, Massachusetts, Ohio, Pennsylvania, and by New York, in which New York had more than three times the population of Virginia by that time. Further to the point, in 1840 the southern States that would eventually make up the Confederacy in comparison to the Northern States (while also including Border States that did not secede to the southern cause) showed that in 1840 those southern States were approximately 31.81% of the American population. In 1860, when secession was first declared by South Carolina in December of 1860, these same southern States were approximately just 27.75% of the United States population.
While there are a lot of reasons why the southern States grew at a far lesser percentage than the northern States, one of the primary reasons, if not the primary one, was the massive immigration of Europeans into America from 1841-1860, in which approximately 4,311,465 Europeans came to our shores, at a time in which the population for the entire United States in 1840 was only 17,063,353 and primarily those Europeans immigrated in massive numbers to the northern States and not to the south. In the coming civil war, those numbers would give the industrial north an impressive additional advantage over the south, an advantage that the north was able to successfully use to defeat the rebellion.
Although there are many logical reasons why Europeans migrated to the north as compared to the south, such as the fact that they often first arrived in more northern ports, most of the time, such as New York, the most basic reason that the north was the chosen destination over the south, has everything to do with the better economic opportunities that the north provided to these immigrants. That is to say, in the north, land was both abundant, available, and cheap, in addition there were plenty of jobs at ports, in manufacturing, factories, mining, as well as the cities of the north having an infrastructure that was more adept at accommodating and sustaining these immigrants.
The fundamental problem that the south had in attracting immigrants was that the south was a society built upon slave labor and not free labor. Further to this point, it was a society to which large landholders in the southern agrarian economy, had the vast majority of political as well as economic power, in which sharing such power and/or making it a policy to accommodate immigrants were not part and parcel of the southern way.
People that voluntarily take the massive risk to come to a land over vast distances do not do so without having the internal fortitude to place themselves into a position to which they can achieve things that were not considered possible in their former homelands. All things being equal, people vote with their feet, and those new immigrants that came here, came with a purpose and a deep desire to achieve great things by hard work and effort not only for themselves, but also to set the table for the advancement of their progeny and further it was these immigrants specific belief that the States of the north offered the better avenue as well as providing more freedom to do so.