The idea of the Superman curse is something that already exists and that might’ve proven tricky to navigate for this crossover, had the episode come to fruition. It stems from several misfortunes involving those who have suited up as the Man of Steel or been involved in the projects, most notably George Reeves and Christopher Reeve. While no one lends much credence to the theory, some might’ve balked at the real-world significance of tragedy befalling actors portraying Superman, and it could have come off as quite callous, depending on how the subject was treated. In what surely would have been a lighthearted episode, I’d like to imagine that was a balancing act the writers could’ve pulled off.
It’s unclear when the idea was pitched, but if it was season 6, which was when Stewart co-wrote “Frontierland,” that would’ve been during the 10th and final season of “Smallville,” so it’s entirely possible no one wanted to mess with the show’s swan song. With both series off the air, we’ll never see how a crossover between “Smallville” and “Supernatural” would’ve turned out. However, Welling did appear in prequel series “The Winchesters” as Samuel Campbell (father to Mary and grandfather to Sam and Dean), so at least he got to dip a toe into that world.