Kirby is more or less a blank canvas for Nintendo to use for there many different ideas.
You want a game were you controls a series of things with the touch screen? Throw Kriby into the mix! Want to have a 4 player racing game using flying stars with open locations? Make that a Kirby Racer! Want a game to use the DS's touch screen to its fullest while being traditional to past games? Make that a dam good Kirby game!
The series, more so than Mario or Pokemon is a franchise that is defined by creativity, with every game in the series doing something different or unique. There are some exceptions to this rule (
Kirby Super Star, Kirby Nightmare in Dreamland, Kirby and the Amazing Mirror, Kirby Returns to Dream Land, Kirby 64 and Kirby Triple Deluxe), but lots of games in the series do something completely unique and in many ways, do a better job showing off Nintendo's systems than the
face of the company.
It also doesn't help that Iwata worked at Hal (the studio that makes Kirby games) and Sakarai makes Nintendo's biggest games (Kid Icarus and Smash Bros.), so that could be it too
.