Well, to put it into perspective, here in Australia, the number of Christians is much lower than in the USA. The reason for this is that Australia was established primarily as a penal colony. Most of the people who then became citizens and governors and soldiers of Australia were ex-convicts, children of convicts, indentured servants and former prison guards. Some people were free settlers. But the majority were here not out of choice. The goal of most people wasn't to turn Australia into a Christian country.
By contrast, the USA was founded by religious free settlers (including Baptists, Calvanists, Methodists, Presbyterians, Quakers etc) who were keen to convert the natives and further their particular denomination by establishing themselves in a completely new (and very large) land, free of any political barriers. They wanted to make America a continent that was a haven and stronghold for their religion, free of the constraints on freedom of religion that they had previously experienced in Britain and the rest of Europe.
Someone living in the US would have to explain precisely why the south stayed more fundamentalist than the east, north and west.
