Atlanta native here.

The simple answer is that there is no particular reason. Atlanta is just a spot on the map. The city was initially known as Terminus - as in, it was the end of the railroad line. The convergence of several major railroads made it a vital link for the Confederate armies in getting the resources of the deep south to the front in Virginia. This was why W.T. Sherman targeted the city and ripped apart its rail network.

After the War, the city reinvented itself as a symbol of the New South (the city crest still includes the Phoenix) and also became the new capitol of Georgia. People like Henry Grady, who founded the Atlanta Constitution, promoted the city as a place to do business and it became a regional center known for its relatively moderate racial politics and for having a thriving black middle class centered around the Sweet Auburn neighborhood. This wasn't always true, however. Atlanta experienced a race riot in the 1906 and the infamous lynching of Leo Frank happened in 1915 (in the suburb of Marietta).

By the end of World War II, Atlanta was a large city, but it wouldn't have been considered the regional capitol it is today anymore so than Birmingham in neighboring Alabama. However, in the 1950s, Hartsfield International Airport was opened and became a major transit hub and one of the world's largest airports, bringing huge amounts of business to the city. Also, while other southern cities often had violent approaches to the Civil Rights movement, Atlanta was pretty accommodating. Martin Luther King, Jr. and his lieutenants were all from the area and his organization, the Southern Christian Leadership Conference was based on Auburn Avenue. Also, white Atlanta was often pro-civil rights like Mayor Ivan Allen, Jr. and Constitution editor Ralph McGill. This all led to a willingness for more northern and international business interests to invest in the city. For example, both the NFL and MLB, which had often been hesitant to move into the deep south, put teams into the city in the mid-60s (the Falcons and Braves).

In the latter half of the 20th century, the city's suburbs boomed, buoyed by strong in-migration from northerners seeking cheap real estate, nice weather, and a good job market. The construction of four major interstates in the city (I-75, I-85, I-20, and I-285) also allowed the metro area to expand in all directions, with no real geographic boundaries to pen in the growth.

The inner city experienced disinvestment and population loss caused by the "white flight" phenomenon, although it has experienced an urban renaissance and gentrification since around the turn of the millennium.

In short, the answer is that Atlanta is a crossroads town, whether it be by rail, air, or highways and people are drawn by the business opportunities to be exploited. Also, the relatively cordial racial relations in the city have made the city a symbol of the "New South."

Sources: Atlanta and Its Environs, by Franklin Garrett. This is the largest (I think it's at 4 or 5 volumes now) history of the city.

Also, for more on Atlanta's complicated racial relations (which are way less rosy then I went into above) there's Where Peachtree Meets Sweet Auburn by Gary Pomerantz.

That's an interesting question. I wonder that about a lot of cities. I don't want to hijack your thread but I'm interested in the reconstruction of the city after it burned as well.

Hey, South Georgian here!

The basic reason for Atlanta's development into a city is that it was the spot of a rail depot where the rail line from the north and Midwest met to travel out west to Alabama or more importantly east to Savannah. The original rail line was built until the early 1820's, and by the civil war the city was a big rail way hub, but still had only about 10,000 people living around it at most. After Sherman burnt it down, and then everything else in the way to Savannah, curse you Sherman!, The rail lines were still relatively intact and the Georgia state Government moved the capital to Atlanta to capitalize on the rail infrastructure that was already there.

While you're right that there isn't a river for shipping, there is the flint river system around Atlanta, and a pet project of some of the Mayors of Atlanta over the last few decades has been trying to open up that system to commercial shipping. I think many cities like Atlanta have developed in america, especially out west, due to being important railway stops, its just less common on the east cost which had been settled longer before the invention of the railway.