Why she's on the list:
Yes, yes, laugh all you'd like. It makes perfect sense that Celine Dion has never come anywhere near Guitar Hero or Rock Band. But then, Rock Band especially has never shied away from straight-up pop, and recent Nickelback and Limp Bizkit DLC packs prove that quality isn't always a major concern.
Dion's certified record sales top 111 million over her career, which makes her one of the best selling artists of all time. Sure, those sales are almost definitely driven by one single song, but you know that one song would be a stone-cold DLC hit, and wouldn't have been out of place on the kitschy and family friendly Lego Rock Band game.
Why she may not be in the games:
Somehow Dion is blander than Wii party games despite her shameless, over-the-top melodrama, and would only appeal to the most mainstream or ironic of Rock Band's audience.