Isaac Asimov (; c. January 2, 1920 - April 6, 1992) was an American writer and professor of biochemistry at Boston University. He was known for his works of science fiction and popular science. Asimov was a prolific writer who wrote or edited more than 500 books and an estimated 90,000 letters and postcards. His books have been published in 9 of the 10 major categories of the Dewey Decimal Classification.

Asimov was born on an unknown day between October 4, 1919 and January 2, 1920, inclusive. Asimov himself celebrated it on January 2.  Asimov's parents were Anna Rachel (nee Berman) and Judah Asimov, a family of Jewish millers. He was named Isaac after his mother's father, Isaac Berman. When he was born, his family lived in Petrovichi near Klimovichi, which was then Gomel Governorate in the Russian Soviet Federative Socialist Republic (now Smolensk Oblast, Russia). Asimov wrote of his father, "My father, for all his education as an Orthodox Jew, was not Orthodox in his heart", noting that "he didn't recite the myriad prayers prescribed for every action, and he never made any attempt to teach them to me".  In 1921, Asimov and 16 other children in Petrovichi caught double pneumonia. Only Asimov survived. He later had two younger siblings: a sister, Marcia (born Manya, June 17, 1922 - April 2, 2011), and a brother, Stanley (July 25, 1929 - August 16, 1995), who was vice-president of New York Newsday.  His family emigrated to the United States when he was three years old. Since his parents always spoke Yiddish and English with him, he never learned Russian, but he remained fluent in Yiddish as well as English. Growing up in Brooklyn, New York, Asimov taught himself to read at the age of five, and his mother got him into first grade a year early by claiming he was born on September 7, 1919. In third grade he learned about the "error" and insisted on an official correction of the date to January 2.  After becoming established in the U.S., his parents owned a succession of candy stores, in which everyone in the family was expected to work. The candy stores sold newspapers and magazines, a fact that Asimov credited as a major influence in his lifelong love of the written word, as it presented him with an unending supply of new reading material as a child that he could not have otherwise afforded. He became a naturalized U.S. citizen in 1928 at the age of eight.

Answer the following question by taking a quote from the article: Why did they move to the United States?
when he was three years old.