Because jazz is one of the few great movements in art that is truly an American phenomenon, it is with great pride that cities that played a pivotal role in the development of jazz like to honor this heritage. In its early stages, jazz grew and changed into different forms as it was popularized in different cities and it is difficult to pinpoint the exact place and time when jazz was born, but it is impossible to tell the story of jazz without the development of Dixieland jazz in New Orleans in the late 19th and early 20th Centuries.
Dixieland jazz originated out of funeral marches where tradition held that a party of musicians would solemnly play a church hymn en route to the funeral in mourning and play a livelier version on the way back in celebration of the memory of the deceased. The marching party would consist of trumpets, trombones, and clarinets, with the possible addition of banjo, a bass drum, tuba, and saxophone. The players generally did not read music. Instead, they were familiar with traditional hymns and as the song was repeated, they would improvise their own variations on that melody and rotate that melody around. This would be a stepping stone to the later tradition of instrumental solos that would be a stalwart of modern-day jazz. The practice of everyone in a jazz community being well-versed in certain standards as well as the emphasis on the horn section (as opposed to the instruments in the rhythm section) to bring out the melody also originated here.
The music was so lively that bands would be hired for festivities, picnics, Mardi Gras time, or even the opening of a new store. In venues such as Preservation Hall, the music developed with musicians and bands making names for themselves through the addition of rhythm sections consisting of non-marching instruments such as stand-up bass, piano and drum set. The role of the piano in particular illustrates how jazz was carried from one place to the next as piano players in New Orleans would come to adopt the ragtime tradition that was originally popularized more by the musicians of St. Louis (where Scott Joplin led the way) and infused it into that Dixieland sound.
As for why this happened in New Orleans, it mostly had to do with two factors: the abundance of opportunity and the diversity in the area. Preservation Hall founder Allan Jaffe said in an interview with W. Royal Stokes that music flourished in New Orleans because it wasn't just for entertainment. It served a functional purpose as well. Because the music was used in churches, funerals, parades and store openings, there was always a promise of a source of income and even respectability for you if you could pick up an instrument and play it well. On his website which tracks the history of jazz, John P. Birchall notes that in New Orleans there was also an available supply of military instruments left over from the Spanish-American War.
Jazz would not have been able to get off the ground either if it wasn't for the diversity of the region either. New Orleans was the most cosmopolitan city in America with the city consisting of blacks, whites, European immigrants, and most importantly Creoles. Creoles are an ethnic group native to Louisiana that resulted from breeding between Native Americans, French settlers and slaves and were perhaps the most affluent group of non-whites in the United States, owning a significant portion of the industry in New Orleans. Their culture was centered on music and they started the traditions of playing in parades and created the dance halls where New Orleans music was bred. Because they enjoyed being a distinct culture, they did not feel the need to emulate European art forms and were far more willing to bring in elements of the music around them whether it was plantation slaves, churches, or immigrants. Because of their affluence, they mingled with the white population of New Orleans far more than would happen in any other city and because some of them worked alongside African-Americans in trying to eke out a living through music and culture, they even served as a mediator of cultural exchange between blacks and whites and made the music more visible.
Of course, the next chapter of the development of jazz would take place in Chicago where many of the major musicians such as Louie Armstrong, Jelly Roll Morton, and Tony Jackson migrated to Chicago and popularized the art form there. Again, one could get away with saying that jazz's birth originated in the Midwest with the syncopation of the piano or in Chicago where the configuration of the modern jazz band was popularized but so much of what made jazz the way it is happened in New Orleans. Furthermore, Americans think of jazz as a homegrown phenomenon not just because it was created within the borders of our country but because its development, through the fusion of different cultures musical styles, embodies the American melting pot ideal. That is why people romanticize New Orleans role in jazz as the place where cultures melted together like never before.