Final Fantasy X hit our shores December 2001 and has since then sold over 1.8 million copies (5.9 million worldwide). Then, in early September 2003, Final Fantasy X graduated to become on of the PlayStation 2 "Greatest Hits" games that retail for $19.99 US. At a price just under $20, everyone could afford to pick it up. Reasons for moving to a mass market price point? Either to jump start sales, or to gear everyone up for a sequel.
Even though Final Fantasy X is the tenth 'title' game in the series, you couldn't really call any of the Final Fantasy follow-ups true sequels. By the American Heritage dictionary's definition, a sequel is "a literary, dramatic, or cinematic work whose narrative continues that of a preexisting work." While many of the Final Fantasy games share certain elements (such as Chocobos and a guy named Cid), the worlds, characters, narratives, even play mechanics were all independent of one another.
News source: Firingsquad Gamers