The main reason why the Southern Baptists are generally wealthier than Baptists are in the North is the fact that the Southern Baptist Convention represents the mainstream of Southern life. The Southern Baptists are not a relatively small sect the way that the Baptists in the North are.
In the South, the Southern Baptists have long been a major church of people who are part of “the establishment.” We can see this by looking at lists of people who are Southern Baptists or who were raised as Southern Baptists. These people include Al Gore and Jimmy Carter. They also include other politicians like Sonny Perdue, who was governor of Georgia and current US Senators such as Lindsey Graham, Ted Cruz, and Tom Coburn. What all of this indicates is that the Southern Baptist church is very much part of the middle and upper class mainstream in the South. By contrast, the Baptist church in the North is not the church of the middle class or the elites. The Baptists are, instead, a relatively small sect that is often popular among people with less money. In the North, it is typically people from the “old line” churches who are in the upper class.
The main reason that Southern Baptists are richer, then, is because their church is a mainstream church in the South. It became an important church as an expression of Southern pride and in that way attracted a richer class of congregants. In the North, the Baptist church was not able to position itself as part of the upper class’s religious experience.