Neil Gaiman Explains Sandman Absence From DC Movie Slate
Other sources
Why The Sandman Wasnt Part of Warner Bros. DC Film Slate
Neil Gaiman hints at 'slate' of Vertigo comic book films
On Tumblr, Gaiman was asked about Sandmans absence in DC movie calendar, and he responded succinctly.
Its not a DC Comics film. Its a Vertigo film. Thats a different slate of films, and a different announcement.
Other sources
Why The Sandman Wasnt Part of Warner Bros. DC Film Slate
Neil Gaiman hints at 'slate' of Vertigo comic book films