The Hours is a 2002 adaptation of Michael Cunningham's 1998 novel of the same name.
Because both the film and the novel deal with the same central character and theme -- British author Virginia Woolf and her novel Mrs. Dalloway -- scenes involving her necessarily need to be set in London, where she lived and worked. The other two protagonists, however, live on the East and West Coasts of the United States, and although the film adapts the book faithfully, it doesn't strictly need to. Keeping the two protagonists in the U.S. serves the purpose of distancing them from Woolf's reality and her way of life; across the ocean, and many years earlier, Woolf dealt with far different cultural norms than in the U.S. in the 1950s and 2000s. Woolf was a real person and for a realistic work needs to be portrayed in reality, in her known residence; the other two are fictional, and could legitimately be transplanted as the needs of production or story require.