There are many reasons for not finishing a book. You may be in the process of reading it and because although you would love to finish it, it is so great that you can't bear the thought of it ending and you prolong the inevitable for as long as possible. Or you might be in the middle of it and decide that although you have given it enough of a chance, you don't want to waste any more of your time reading it. Or you have have read the first few pages to kill some time and days later after your busy rush is over, you spy it on your bookshelf but can't bring yourself to go over and fetch it. This is the worst, because although the story line possibilities will plague your imagination, you will return it among your fully read library books.
I know of this latter one because right now I am thinking about a library book which i returned a few days ago. This was when I had in fact been sitting on my couch contemplating reading it and across the room upon my shelf was that very book and because I didn't, my mind has been a-whir thinking of all the possible plots and story lines. My verdict, I think I shall take it out again.