(To this day, even. Over fifty percent of American voters say they would not vote for a non-Christian)
Depends on how the question is phrased, really.
If the OP is asking, "why is Christianity so heavily associated with 'white' in the US," that's because of slavery and segregation and whatnot. Slavery was actually a big battle between two different kinds of religious thinking. One was the heir to the Puritan throne (in the North) which sought to end slavery in order to "purify" the US, while the other was a more Evangelical-type faith (in the South) which thought that faith could "save" the slaves, but only if they stayed in bondage and were therefore forced to receive the teachings.
Today, these each manifest as, respectively, religious fundamentalists, and television evangelical bible-quote mongers.
If you want to know why white = Christian around the world, I think it's been effectively answered...