Why does the book The Giver have so many unexplained scenes?
Although I am sure you are at least referring to the ending, as that has always been up for debate, it is difficult to know which other "unexplained scenes," you are referring to. Although Jonas is not always aware of why something happens the way that it does, as a reader we generally do not leave most of the scenes feeling as though there are loose ends - except the ending. So, let's look at that.
Of course, the book ends with Jonas and Gabriel on a sled sliding toward a house with what appear to be Christmas lights. There has always been much debate about whether or not Jonas and Gabe live or die at the end of the novel, since Gabe appears to be near death (at least) just before Jonas sees the house.
Although Lois Lowry has said in interviews that she was stunned by the initial reactions and confusion with the ending of the book, she has also said it was purposefully written to "make people think." In other words, while she may have bene surprised by the debate, she was also happy about it. She liked that it was not neatly wrapped up. This is frustrating for readers of any novel because we all like to know how the story ends, and when we do not get a clear picture of that, we feel as though we are missing something.
Ultimately, Lowry did admit she never planned to write a sequel to The Giver, but ended up doing so in response to the debate about the end of the first book.