In the episode where he was mentioned, Bud told Kelly that Al attributes Stymie becoming rich despite being a Bundy to the fact Stymie never got married. However, in a later episode, Al mentioned a "Bundy Curse" that makes Bundys lucky until they admit they're lucky and then the bad luck begins. It's possible that Stymie, unlike the other Bundys, was Genre Savvy enough to simply refuse to admit it when the "curse" brought him good luck. The never got married thing could be either an additional cause or an additional consequence.