This is a great question. Elie Wiesel does not tell the reader plainly why he called his book, night. However, there is one passage that shows how powerful and painful his first night was behind barbed wires. According to him, he will never forget what happened that night. In view of this, he probably called his book, night. To put it another way, that night made all his life into one long night. He is what he says:
Never shall I forget that night, the first night in camp, that turned my life into one long night seven times sealed. Never shall I forget that smoke. Never shall I forget the small faces of the children whose bodies I saw transformed into smoke under a silent sky. Never shall I forget those flames that consumed my faith forever. Never shall I forget the nocturnal silence that deprived me for all eternity of the desire to live. Never shall I forget those moments that murdered my God and my soul and turned my dreams to ashes. Never shall I forget those things, even were I condemned to live as long as God Himself. Never.
If we parse these words, Wiesel is saying that that frist night represents suffering, death, crisis of faith, and sheer disgust. He has seen, experienced, and lived through all of these things. In other words, he saw night without day, where hope was destroyed. Hence, that night refers to his first night, but also figuratively the lack of light or faith in humanity and his God.