The Program
Five of the six men involved in the contest all, more or less, had a legitimate claim to the championship around Helmsley's waist, a serious case of bad blood with its holder, or ongoing tension with a man who had a claim to the title.
HHH stopped a surging RVD with a combination of a Pedigree and interference by manager/mentor Flair at September's Unforgiven PPV event, then engaged in a bitter feud with Kane over tasteless allegations of lewd and lascivious conduct with a corpse unifying the World and Intercontinental Championships at No Mercy (with the IC Title, thankfully, making a return in the following year).

Cody Rhodes' uncle is a WWE Hall of Famer. More details HERE.
Chris Jericho and Christian, meanwhile, had defeated Kane and his partner Shane "Hurricane" Helms for the tag team championships in October. Booker T, meanwhile, was present.
The most compelling storyline, though, was the story that had been told since the preceding spring: that of the return, rebirth, and resurgence of "The Heartbreak Kid" Shawn Michaels, who warred with HHH in what seemed to be the culmination of 2002's standout feud at SummerSlam.
Michaels shook off over 50 months of ring rust after a broken back forced him to retire upon passing the WWF torch to Steve Austin at Wrestlemania XIV, turning in a bloody, brutal, and breathtaking performance where Michaels performed his high-flying spots like he'd never left (and in jeans, no less!).

That match had been teased as a one-time-only unsanctioned return for HBK, but Bischoff announced that Michaels would be the sixth man in the new Elimination Chamber Match; Shawn immediately made his presence known first by costing HHH a casket match against Kane, then a tag match pitting Helmsley and Jericho against Van Dam and Booker T.
While six men were involved in the contest, the writing on the wall was that this would be a continuation of the feud between Helmsley and Michaels, with four other guys to crank up the spectacle.