Science-Fiction has been a staple of feature films since the beginning of the medium, and there's a reason for that. They tend to rake in a ton of cash.
Granted, not every sci-fi movie does well, but when the top-grossing of movies of all time include the likes of Star Wars, Avatar, and Avengers: Endgame, it's no wonder the genre is well-represented in Hollywood.
Still, despite having some of the highest-grossing films of all time, sci-fi movies aren't a guaranteed hit, and over the years, movies have come out that proved themselves the exception to the rule. When a film flops, any planned sequels often never see the light of day.
It happens more often than most people realize, and movies that languished in Development Hell for years may have come close to clawing their way out, but never managed to actually do it. Studios trash projects all the time, and nothing is guaranteed.
Of course, there are those movies that look to be all but guaranteed right up until the studio pulls the plug. These ten sci-fi movies were all set to tread their way out of Development Hell, only to wither and die, never to be released... at least, for now...