Atlas Games' Lunch Money is an example of what could have happened to Munchkin if Steve Jackson Games hadn't followed a savvier marketing strategy.
Lunch Monday came out in 1996 and sold great immediately. Who wouldn't want to play a game in which you're smacking around other people and taking their lunch money? Interestingly, besides Dungeons & Dragons, it's the only game I've every had people complain to me about carrying because they found it morally objectionable. We used to sell an average of a copy a week at the store and usually 3-6 copies when we set up at a convention.
Unfortunately, that was it for the game until 2004, when Beer Money and Sticks and Stones came out. Despite the fact that Sticks & Stones is an expansion deck for LM and Beer Money mixes in with LM, neither sold as well as the original game. Why? My thinking is because Atlas didn't follow the more successful Munchkin strategy of Steve Jackson Games.
Both started out highly popular. However, Lunch Money released 14 years ago and only has 2 additions, plus t-shirts and some greeting cards. Munchkin released in 2001 and currently has 8 stand alone variants, 14 expansions for said variants, plus add on merchandise such as t-shirts, counters, dice, dice bags, even a bobblehead. All of this in less than 9 years. We sell at least one Munchkin game or expansion a week. We haven't sold any Lunch Money at the store or a convention in over 6 months.
Both are good games but Munchkin keeps coming out with new products, that, even if they don't sell as well as the original game, attract peoples' attention and get them talking about and anticipating the next release. There's nothing to talk about, no buzz surrounding any new Lunch Money release. There wasn't even an anniversary edition released in 2006. Want to bet we see a Tenth Anniversary edition of Munchkin in 2011? Yet another item for Munchkin fans to talk about and want to see and likely purchase.