She's the only American Idol winner whose actual packaging ended up being marketable. She ended up a pop singer, and she's stayed a pop singer. On the other hand, both Ruben and Fantasia ended up being suited for markets that simply are impossible to market. Sure, they both made their initial "Let's ride the wave of Idol success" records, but after that Ruben put out a Gospel album that bombed, and Fantasia's album was a relative flop.
Clarkson, through two albums now, has managed to create stable and catchy pop music that the kids dig. Her voice is good enough to pull it off, she's still wholesome enough for the parents to approve, and she actually has a career ahead of her, even if it isn't one that will be bringing her Grammys.
Yeah, and her ass is kinda nice. "Since You've Been Gone" is catchy, if derivative.