It's kind of a result of New England culture pervading the rest of the continent as the country grew. New England was founded by Separatists (starting with the Mayflower) and Puritans. Puritans wanted to "purify" the Church of England of perceived corruption. Seperatists had written off the church as a lost cause, which was treason subject to the death penalty.
A trickle of Separatists and Puritans settled New England from 1620 to 1630. Then from 1630 to 1650 many came over during a period called the Great Migration. Then the migration abruptly stopped (it stopped being uncomforatble to be a Puritan in England). So the culture stewed in its own very, very doctrinaire juices, and became homogeneous. Then it's ideas spread as the country expanded. Among those ideas is never using a word that is not written in the King James Bible, much less a swear word. Of course, there isn't a lot of logic to that choice of vocabular no matter how you analyze it, but there you are.
The Puritan church died out, but the no-drinking, no-dancing, no-card-playing version of the Congregational church survived quite nicely intact to my father's generation. (It is possible for it to wear off -- as my friends with whome I have played a game of Hearts over a few beers can attest. I'm a lousy dancer, though.)