Universal passed on the project--which would have been made up of three feature films and a pair of miniseries--earlier in the year after having cast Javier Bardem in the lead. The reason given at the time was that the budget and scope of the project had gotten out of control.
The Warner incarnation was rumored to have Russell Crowe, who has worked with Howard in the past and will star in this year's Warner Brothers tentpole Man of Steel, under consideration for the role. Warner was the obvious place to go after Universal passed, because writer Akiva Goldsman, who wrote the script, has a relationship with the studio.
While it was always viewed as a long shot that another studio would pick it up after Universal declined to do so, Warner Brothers seemed like as good a shot as there would be, given that they've recently lost the Harry Potter franchise, Christopher Nolan's Batman trilogy and will soon be making the last of the Hobbit movies, meaning that their three top-grossing franchises are all gone, and the studio logically should be looking to replace them.
The Dark Tower series consists of eight books and a series of interrelated comic books; Stephen King has described it as his "magnum opus," and the Peter David/Robin Furth/Jae Lee miniseries that Marvel published have been among their best-selling licensed comics in years.