Films take a lot of money and manpower to make. The average Hollywood film takes $125mil to make and only about 250 movies are recognized by Hollywood every year. With those numbers working against the average creator, there's very little wonder that so many films never receive the sequel or franchise that those involved had planned in their head.
Luckily, comics are cheaper and usually less reliant on needing to make the annual national budget of Greece to have a job the next day.Developing a comic book off of an existing script or idea is not a hard thing to do, with the biggest issue coming down to licensing.
Some creators use their existing ideas and shape them to fit outside of the film they were working on, such as Alejandro Jodorowsky who spent years working on the Dune film adaptation before that fell through, so he adjusted the story notes into what is now called the Jodoverse of comics.
The few who are able to save their sequel ideas through proper application of writing and rights are the luckiest few of all. They're so few and far between that we couldn't even get a full ten for this list!
Learning to read from his parents' old comic book collection, ink has flowed through his blood since he was born! And now, the self-proclaimed Comic Archivist (at least on Youtube) brings his talents for comic book history and obscurity to WhatCulture! Be warned: he's a Cyclops fanboy and a Nightwing groupie!