It was because Mars U.S. and Mars U.K. were separate companies.
Frank Mars and his son Forrest didn't get along very well at all. Frank invented the U.S. Mars Toasted Almond bars, but Forrest claimed credit for inventing the U.S. Milky Way. Forrest wanted to expand internationally, but Frank wouldn't hear of it. So Forrest went over to England on his own to set up an independent European operation that he wholly owned.
After his father died, Forrest inherited the U.S. company, and he ultimately merged the U.S. and European operations -- but he retained the existing branding differences.
http://www.independent.co.uk/arts-entertainment/life-on-mars-the-mars-family-saga-has-all-the-classic-elements-1535722.html
"In 1932, Frank gave his son dollars 50,000 and the foreign rights to the Milky Way.
"With his wife, Audrey, Forrest travelled to Europe. In Switzerland, he worked for the Nestle family, learning to make European chocolates. Ultimately he settled in Slough, just outside London - a small industrial town where he felt comfortable and could speak the language.
"Like his father before him, Forrest set up shop in a one-room factory. He started out with an anglicised version of the Milky Way. A little sweeter than its American counterpart, it contained only the finest ingredients, a lesson he took from his father, and he named it the Mars Bar. It wasn't long before Forrest's bar became a bestseller."
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Forrest didn't inherit the U.S. company after Frank died; he had to purchase it from his father's antagonistic second wife.
From the same article:
"When Frank Mars died in 1934, at the age of 50, his wife's family had assumed control of the Chicago plant [Mars U.S., which had Mars, Milky Way and Snickers for the U.S.; M&Ms were created by Forrest during WWII and always belonged to him] and refused to give Forrest a stake. For several decades, the two families fought a bitter battle for ownership. Finally, in 1964, her family decided to sell out and, at 60, Forrest returned to take what he claimed had always been his."
