Wario Land: Shake It! director Madoka Yamauchi came up with an idea for "World of Fluff" and designed it around the Prince Fluff character. Nintendo loved the idea, but thought something was missing from the "warm" world Good-Feel had created. Nintendo proposed turning it into a Kirby game last year.
Producer Etsunobu Ebisu said that having Kirby in the world gives Epic Yarn "a greater feeling of existing." Also, we're sure, Kirby gives it a greater feeling of hope for good sales.