During the 19th century of the antebellum era in America, European immigrants came in waves of somewhere around 100,000 to 400,000 peoples per year and while the most popular debarkation point was the great harbor of New York, there were also ports available for European immigrants in Boston, Baltimore, New Orleans, and others. At the turn of the 19th century, the most populous State of the Union was Virginia, yet, by 1850, New York had nearly three times the population of Virginia, Pennsylvania was doubled the population of Virginia, and Ohio had nearly 75% more peoples than Virginia. This meant, in a relatively short period of time of about 50 years, the North and Midwest portions of the United States, had gained a considerable number of people, whereas the South, while growing in number, was in aggregate falling further and further behind. The biggest factor in the population growth of these sections of America was the great wave of European immigrants that came to our shores.
While certainly there is an advantage in being the port of choice, as in New York, immigrants aren't stupid, as the most basic point of them immigrating to the United States, was to take advantage of the opportunities that the country offered them and for that, they would as a matter of course, go to where they perceived the opportunities were most favorable in regards to employment, land, skill-set, and so forth. It is in this regard, that the North and the sparsely settled Midwest offered far more basic incentives for European immigrants than the South, as for instance, the Midwest had plenty of open land, available for agricultural and for homesteading, while the Northeast was the great manufacturing center of America, ideal for skilled artisans and the like. The problem that the South had was that the distribution of their land was held in very few hands that utilized as a matter of course, slave labor to enrich their lands and themselves, so that many Europeans would not have a great interest in trying to compete against interests that favored the ruling class, as well as trying to compete with their free labor against slave labor.
Had the South been more accommodating to European immigrants, the history of the States, might well be different, but because the South was setup essentially as a feudal republic, this closed society, fundamentally limited itself to land assets and the fruits of their lands, as opposed to mechanization and the industrialization, as well as the education which were all part and parcel of the Northern States. This meant that the European immigrants that came here, highly motivated and delighted by the economic opportunities now available to them if they applied themselves in the meritorious North, as well as appreciating the basic freedoms of religion, assembly, and speech gravitated to the States of the Union that did not or were in the process of eradicating legal slavery from their territory.
The obvious thing about the economics of slave labor, is that if you as a free man have only as assets your own hands and the skills that you bring to your labor, and those that would employ or need your services have the choice of paying you a fair wage or taking labor from the sweat of the brow of a slave, than they will do the latter, and you, as a free man will find somewhere else more accommodating to ply your given trade.