Why is a 400+ year old systematic theology so controversial? The fear of non-Calvinists is that Reformed Theology is going to cause a decline of evangelistic and missionary zeal from Southern Baptists.
Here are 4 reasons for the popularity within SBC churches:
1). Wayne Grudem's Systematic Theology book. If you go to a SBC seminary, you're going to own and read this book. General atonement believers don't have a mass produced text like Grudem's. His beliefs would be considered Reformed Baptist - a clear, well-organized, and well-published textbook that has shaped a new generation of SBC pastors.
I'm a 3 point Calvinist (Irresistible Grace and Limited Atonement aren't scriptural) - but do reference Grudem's book frequently because its ease of use.
2). John Piper. Its amazing how a General Baptist Conference pastor in Minneapolis, Minnesota has had such an impact on Southern Baptists. Piper has written hundreds of books, articles and headlines large conferences. His resource website, Desiring God, provides over 5,000 resources - a toolbox for any church leader. John MacArthur, Mark Driscoll, R. C. Sproul, and Timothy Keller have all been influential in SBC life.
3). The Southern Baptist Theological Seminary. The SBC's flagship seminary has key leaders in SBC life who adhere to Calvinism: Albert Mohler, Don Whitney, Mark Dever (trustee) and Tom Nettles (who wrote a book stating that James P. Boyce, the first president of SBTS, was a Calvinist).
4). The Young, Restless, Reformed Movement. In September 2006, Christianity Today published a must-read article labeling the rise of Calvinism. This "new Calvinism" of today is more hip, cool, trendy, and appeals to 20 & 30 year olds.
Is Calvinism Biblical?
Whosoever Will: A Biblical-Theological Critique of Five-Point Calvinism
Founders Ministries (the pro-Calvinism group within the SBC)
SBC Today (the anti-Calvinism group within the SBC)