I think that's because the term "Oxford comma" is somewhat of a misnomer. The comma before "and" actually seems to be more preferred in the United States that in Britain.
"Opinions vary among writers and editors on the usage or avoidance of the serial comma. In American English, the serial comma is standard usage in non-journalistic writing that follows the Chicago Manual of Style. . . . It is used less often in British English,[5][6] where it is standard usage to leave it out, with some notable exceptions such as Fowler's Modern English Usage."
"Opinions vary among writers and editors on the usage or avoidance of the serial comma. In American English, the serial comma is standard usage in non-journalistic writing that follows the Chicago Manual of Style. . . . It is used less often in British English,[5][6] where it is standard usage to leave it out, with some notable exceptions such as Fowler's Modern English Usage."
http://en.wikipedia.org/wiki/Serial_comma