The Southern Baptists are one of the only protestant denominations these days having lots of children. The Methodists, Presbyterian, Episcopals, etc are not procreating at the same levels. In the liberal sectors having two kids is having a large family; having more you run the risk of being labeled a "breeder" and being anti-environment among other things. To remain liberal lap-dogs like the main-line churches have become, you cannot have more than to kids. Folks, forcing (key word here) families to remain at two kids is not replacement level, it is not even zero growth. It is negative growth.
The other reason mainstream churches are losing members is the fact that they offer nothing but the "I'm ok - you're ok" philosophy and everything is permissible. The liberal denominations are turning out totally licentious youths who think, "well, if I can find the truth in myself, why do I need church?"
Carter and Clinton are creating what will eventually be considered the real Souther Baptist denomination. The conservative SBC will be considered renegade and be looked at even lower in the eyes of liberals if that is possible.