The current theory is the studio decided to commit its budget to making Battleship and 47 Ronin. My theory is Comcast, whose bad reputation is well earned, likes to play things extremely safe and on the cheap as much as possible. The Dark Tower was going to be very expensive with no guarantees of success along with source material that isn't exactly movie friendly so the concerns are valid.
The main problem is that most studios loath to take "used" projects, especially ones as public at The Dark Tower. While it is possible it could be shopped to another studio, I suspect this will be the end of any hope of adaptation for at least a few years, if ever. I will keep the blog open for a while long just in case things change but to be blunt I don't have much hope.
Update: Stephen King spoke with Entertainment Weekly about the cancellation.
“I’m sorry Universal passed, but not really surprised. As a rule, they’ve been about smaller and less risky pix; maybe they feel it would be better to stick with those fast and furious racing boys. I bear them no ill will, and trust Ron Howard to get Roland and his friends before the camera somewhere else. He’s very committed to the project.”