See the statistics - New York was the largest city almost since the beginning of the United States. The construction of the Erie Canal had a large significance in increasing New Yorks already-strategic location as a port. It's not only the largest city, after Boston it's the closest major city to Europe. After the completion of the canal, it passes Philadelphia and Boston as the main port on the Eastern Seaboard. Its growth started to outpace other cities even more rapidly as it became the main point of immigration, with stations at Castle Garden and later Ellis Island.

Millions upon millions of immigrants pass through there in the 19th century, many of whom don't get much farther than that and take up residence. Between 1890 and 1900 alone, the population doubles. By 1940 it's at 7.5 million, after which it stayed relatively stable (with periods of decline) since, as immigration waned. -The proportion of foreign-born in the US reaching a bottom level in the 1970s. While immigration has increased since, people don't arrive by boat anymore and there are far more ports of entry for immigrants.