Per Wikipedia:
Joseph Park Babcock, a representative of the Standard Oil Company in Shanghai, was importing mahjong sets to the United States in great numbers by 1923. To increase interest in the game, which was sometimes impenetrable to Westerners, he rewrote and published new, simpler rules that became the American standard. When the National Mah Jongg League, Inc. published a volume of "Official American Rules" in 1935, the American style further morphed into a very distinct form.