The Black Cauldron had been in development at Disney since the early 1970s, when the studio optioned the rights to Lloyd Alexander’s The Chronicles of Prydain series.
That also meant the animation was going to be a lot more complex.
In addition, it was felt that the film could be a turning point for Walt Disney Animation from a style perspective, and that it could potentially be attractive to a wider and potentially more mature audience.
As the book series covers numerous characters and plotlines over a long period of time, it was difficult for the story department to find the right angle for the plot of a single film.