Harry Enten believes it's because "there is greater untapped southern talent for American Idol to showcase":
It will come as no shock to anyone living on the coasts that there are many record labels in California and New York. California has about 2,500 record labels or 1 record label for every 15,000 residents, while New York has about 1,500 record labels or about 1 record label for every 13,000 residents. It's fairly easy for a talented musician to get noticed fairly quickly in these regions.
Southerners just don't have as many opportunities. The southern hub of Georgia has only 293 record labels or about 1 record label for every 33,500 residents, while Texas has only 559 record labels or about 1 record label for every 46,000 residents.