You have your answer in your last sentence. A movie is not made for the relatively few who read any book. Movies are made for masses. I'd bet the great majority of the movie going public has little to no idea what the hell The Dark Tower even is, in micro or in macro. We're not talking LOTR here. That level of cultural consciousness could happen some day. But it ain't here now.
Filmmakers perspective: Ron Howard does know and love the story, but even with his clout could never get it financed. BUT he's (his company, anyway) done some of the prep work, right? Now producer #2 (or whatever) comes in. Maybe he/she likes the story, too, or maybe not. Either way, they know that Howard has generally good instincts. So they look at what's been done and make decisions based upon getting the damn thing made. They might or might not have the resources to dick around with it for years.
They need a script that doesn't have to be a years long saga (those aren't doing too well anymore--moviegoers tastes have changed), but can be expanded if it's a surprise hit. So the books are mined for good visuals and a few iconic scenes, then the rest is filled in.
Now you need a star. Can't be a huge expensive name (not without huge money, Oscar tear jerker written on it, or someone willing to work for scale), but it has to be someone who has piqued interest lately. In this case, it was Idris Elba. Back him up with a known quantity (McC), and you're off to the races.
Movie making 101.