Could you explain what exactly you are asking? There is not really a question in your submission.

The Garment District of mid-town Manhattan was the densest concentration of shops that made ready to wear clothing in the world during this time period. It started before the Civil War, when several dozen companies began to sell ready made clothing to Southern plantation owners. They wanted their slaves to work in the fields, not waste time making clothes. The business expanded during the Civil War, when the Union Army bought most of their uniforms from the Garment District. It expanded exponentially in the 1865 to 1898 time period, as a more urbanized America became more likely to buy ready to wear clothing in a store. By 1910 70 % of women's clothing and 40 % of the men's clothing sold in the United States was produced in the Garment District.
The massive influx of German and Eastern European immigrants provided a cheap source of labor. The fact that New York City also established itself as the Cultural Capital of the United States during this time period saw a dense concentration of fashion designers in New York City. There was also an extensive national distribution network that allowed the companies in the Garment District to supply wholesalers and department stores throughout the nation.
Source: "Triangle: The Fire that Changed America" by David von Drehle