Stephen King's The Dark Tower series spans time and space and alternate dimensions and even makes grand marches into meta fiction (Stephen King himself is a minor but important character). To adapt it adequately would be nearly impossible.
Ron Howard has been trying to get this one off the ground for years as a multi-film, multi-television series so ambitious and unprecedented that no studio has yet been brave enough to get fully behind it. If they ever get the adaptation off the ground, just over half of it will be awesome.