It comes down to the money, doesn't it? A book makes money, ergo a film based on it will also make money. A lot of the time, it works. Even more often, it doesn't.
Some books are unfilmable, end of. Dune is a huge novel in which every single action and piece of dialogue fits precisely into place - miss one nuance, and you ruin the whole thing. Impossible to film successfully. A properly-made, faithful, multi-season TV show might just be able to do it justice, but it would be very niche and a financial disaster.
By the same token, LOTR could only be done justuce on film. It's too big, too sweeping to be broken into hour-long TV episodes.
Harry Potter, on the other hand, would do better being given the GoT treatment. Seven books making seven mini-series over seven years would allow the young characters to age properly and for some of the minor incidents and plot arcs that give the books their charm to be included, rather than excised as they were in the films.
Given the option, most living authors would like to have input on films made from their work. JKR resisted the studios for a long time because she was afraid they would Americanise her work (Disney would certainly have done so, and the concept of a blonde Hermione singing to animals musdt have haunted her nightmares). It was only because the property was so desperately hot that Warners allowed her the input they did on the filming.