Talking About Stephen King: Why was the film version of Stephen King’s ‘Dark Tower’ not based on the first novel in the series, ‘The Gunslinger’?

I haven’t seen The Dark Tower, but from what I have heard of it, I think the four screenwriters (Nikolaj Arcel, Akiva Goldsman, Jeff Pinkner, and Ander Thomas Jensen) and the film’s director (Arcel) may have been forced by certain considerations to adapt the Dark Tower series the way they did.
First, as  Quora's Matt Reda points out in his answer, Stephen King’s series is made up by eight novels; seven of them are the main series, while The Wind Through The Keyhole (which King says is Book 4.5, and fits between Wizard and Glass and The Wolves of the Calla) is a side jaunt written after the series ended with Book VII: The Dark Tower.
This is a huge story, as big, say, as J.K. Rowling’s Harry Potter series and almost as big as J.R.R. Tolkien’s Middle Earth mythology, which would include The Hobbit, The Lord of the Ring, and The Silmarillion.
That’s a lot of story to tell!
Ideally, what the four companies involved in adapting this truly massive epic story (MRC, Imagine Entertainment, Weed Road Pictures, and Sony-owned Columbia Pictures) should have done is avoid a theatrical movie altogether and done this as a television series. Not for CBS (like Under the Dome) or ABC (like most of Stephen King’s pre-21st Century movies-for-TV or miniseries events).
Rather, they should have gone for either HBO (which has a great deal of success with long-form drama) or even streaming services such as Hulu (so long as they released Blu-rays/DVDs for those of us who don’t “stream” yet).
You know, like THIS Hulu “event series.”
From what I know of The Dark Tower, the creative team behind the 2017 film intends to take The Dark Tower into the television series realm.
But in order to do that, the powers-that-be decided (perhaps because talk of a Dark Tower movie has been going on since, forever) that they could pull off the same trick that George Lucas sort-of pulled off with Star Wars: The Clone Wars: prepare for a TV series, but whet viewers’ appetite by doing the “pilot” as a theatrical release.
Notice that I said “sort-of.” Star Wars: The Clone Wars was not originally going to be introduced this way; Dave Filoni and Catherine Winder were given the go-ahead when Lucas saw the footage for the first three or four episodes of the TV series and decided that maybe they should get a theatrical release. Lucasfilm and Warner Bros. did exactly that…and they got a lukewarm reaction from fans (particularly the older males that also seem to pooh-pooh the Prequel Trilogy). Some fans liked the film; others thought it was too kiddie-friendly and not like the six-Episode Skywalker Saga movies. The series, though, got better reviews than the movie.
Again, I have not seen the 2017 film, but from what I gather, many King fans dislike the film because - no surprise there - it is not what they expected, i.e., a faithful adaptation of Book One: The Dark Tower: The Gunslinger.