It happens always the same with movie adaptations. Since it is an adaptation, the audience already knows the subject, the story line and even the ending from the very beginning. If it is based on a story with a tragic ending, i.e. the main guy dying of cancer in the end, the couple splitting up, or the mother losing her children, the effect becomes much intense. Though the ending is very tragic and quite unbearable, some audience is still very much willing to go all the way... Watch the movie, get into the feeling, taste all details within the sequences and towards the end feel the tightness in the chest that it's gonna end up badly.
What makes those movies still attractive to the audience is probably the experience throughout the movie. The tension created in the end is nothing compared to the pleasure of living the scenario, seeing all good scenes, listening to nice dialogues, discovering all characters and the music...
But, are all these worth experiencing the trouble in the end? What is even harder is that watching that movie is done willingly... Willingly to go through all that pain to see the tragic ending.
What my personal experience tells me is that living through the best moments of the movie is worth shedding all the tears to the ending. And it is a fact that there will always be other new movies with known tragic endings that the audience will fancy watching, over and over again...