The Northeast of America has always had a somewhat adversarial relationship with the rest of the country. Further west, the Non-Denominational Church has been essentially rejected. In the south, the Church districts are farther and farther apart, and nations pay less and less attention to what the Supreme Court has to say. Religion has been relatively unorganized and independent, and typical evangelist congregations have been supplanted by Voodoo along the Gulf Coast.
There are a couple of other reasons for the spread of Voodoo. Besides the growing infusion of Caribbean culture, the more African descended population may have felt more of a kinship than religion dominated by the ethnically different Yankees. Its spread also has a lot to due with the presence of Secretarial States. Women are given a prominent role in the bureaucracy and stability of the deep south, because the men are off fighting, fishing or doing merchant work. With males usually away, women were also left to their own devices in the field of worship. As Louisiana Voodoo is a faith that gives women a prominent role, it seemed like a pretty natural fit. Spiritual leaders called Voodoo Queens can provide the spiritual needs of the community. It's also a reflection of the Non-Denom church performing the bureaucratic services of its territory. Since the bureaucracies of the Gulf Coast are made up of women, women might as well be helping out with the religious duties. Schools are taught in Creole, medicine is administered by priestesses, and whatever passes for Voodoo organization keeps track of will and deeds. Voodoo Queens may also play a part in the royal family's linage.
Because property is passed from father-in-law to son-in-law, there's less of a pressure to produce male heirs. If the queen gives birth to a boy, that's great, and makes things a little more straightforward. But if it's a girl, she's simply trained to be a High Queen, and they look for a worthy bridegroom among the noble families. Heritage is matrilineal anyways, and this allows the father's direct line to at least play some part in the authority of the state. This of course, has led to a lot of politicking and abuse in its own right, but the point is, nothing short of infertility will cause a succession crisis.
By the way, the presence of Voodoo Queens in the royal family make these states, in theory, theocracies, but the very informal nature prevents secular and religious conflicts of interest. Voodoo leaders are more or less charged with organizing and performing rites, or divinations. There isn't a particular lot of dogma to create schisms with anyways. But that's not to say rulers haven't used to to their advantage.
In some ways,Voodoo does resemble Catholicism, with the importance of Christ and much of the pageantry. However, the Voodoo Church is in many ways quite divergent with the Non-Denominational Church, and only moreso as the two faiths have canonized different saints over the years. Marie Laveau is there most important figure. Voodoo also uses widely different symbols (aside from the cross), with imagery like snakes, skulls and card motifs. Combined with the language, rites, and prominence of women, Non-Denoms view Voodoo practitioners as little more than pagans, and nations like Mississippi and Red River have found themselves embroiled in furious religious wars for Louisiana.