Take a look at this bar graph and note the differences in primary affiliations by state.
http://publicreligion.org/2015/03/top-t ... zOhrRUrKb8Iowa is only one of two states where White Mainline Protestants have a very slight lead over the next two top groups.
A big chunk of US states have either the Roman Catholic Church as their top religious group while another big chunk has "unaffiliated" the largest Christian group. In 17 states (if I counted correctly) White Evangelical Protestantism is the largest group.
What that says to me is that the SBC and other White Evangelical Protestants are likely more easily heard and more known in those 17 states than quite a few of the others. The majority of those states are in the south.