It is terribly difficult to adapt any novel for the big screen (Witness the botch job on John Irving's books.) When there are multiple books e.g LOTR, Potter, Repairman Jack and the Dark Tower series, adaptation costs can be extraordinarily high, locations difficult to shoot in, tax incentives become of paramount importance, finding the right director for continuity (with The Potter films, there have been only 4 directors with David Yates directing the last 4, Jackson on all 4 Hobbit films), finding actors that are appropriate to the role and who can/do/remain throughout the proposed 3 films (this is harder than it appears) and utilizing the same crew and cast throughout if possible-again for continuity purposes except when necessary to change.) Having producers understand and appreciate the books yet realize some modifications will have to be made, not wholesale changes (The best book to film adaptation I have ever read and seen was an Australian film Careful He Might Hear You. The film hewed veryclosely to the book and yet one didn't have to read the novel to understand what the child endured. It was as if the book wrote the film and the film wrote the book.)
And so often a studio missteps in bringing beloved characters to the screen which upsets the book's fans. We all have in mind who should be this fictional person, how certain scenes should be done and how every thing needs to be included. We are all aware of actors being cast because they are cheap or because they can open a film or have a big overseas following! The latter sometimes becomes the overriding concern! Sometimes a trilogy is planned and then the first film underperforms (Green Lantern, anyone?) and no others follow.
If the studio were willing to work closely with the author and that's a BIG IF! and the lead roles cast with actors who endeavor to fulfill what is being asked of them, then the film can be successful.
I can only hope that whenever Jack is brought to the big screen, that it performs sensationally (earning back its negative cost and marketing budget first. Then turns a profit) so that other books may be adapted.