With all the superhero movies coming out nowadays, there’s a lot of money to be made. The Avengers alone made $1.5 billion worldwide according to IMDb, while franchises like Spider-Man have made a habit of breaking box office records. Way back before they were cash cows though, Marvel saw their various superheroes as commodities to be sold off to make a quick buck. Starting as early as 1985, franchises were offered up to studios like Sony, 20th Century Fox, and more, culminating in a whole mess of reasons why Marvel can’t bring characters they themselves created into the cinematic fold.
Each studio with a Marvel property keeps it jealously guarded, knowing full well the moneymaking potential each respective franchise represents. While it would make sense for the rights to return to their original creators, the current owners have kept an iron grip since they first acquired their various properties
Source: 9 Superheroes That Marvel Does Not Own the Rights to Anymore