Why is Room told through Jack's eyes?

Great question! In any story, understanding the author’s choice of point of view is vital because it determines how a reader digests information about the plot, setting, and characters. The point of view also impacts how a reader relates to the characters. Some authors employ unreliable narrators because it forces the reader to work a bit harder to determine what is happening and whether to trust the information provided by the narrator.
Emma Donoghue’s novel Room is narrated by Jack, a five-year-old boy who has lived the entirety of his life locked in a small room with his mother. Unbeknownst to Jack, the pair are being forcibly confined in a basement by a man who regularly rapes his mother. Jack knows nothing of the outside world except what he sees on television. His mother allows him to believe that their small room is all that exists, and Jack lives the first five years of his life without exposure to the real world. Through it all, Jack maintains innocence and curiosity about the blurred lines between reality and fiction. Examine this lengthy passage from the opening chapter of Room to see how this innocence functions.

Mountains are too big to be real, I saw one on TV that has a woman hanging on it by ropes. Women aren’t real like Ma is, and girls and boys not either. Men aren’t real except Old Nick, and I’m not actually sure if he’s real for real. Maybe half? He brings groceries and Sunday treat and disappears the trash, but he’s not human like us. He only happens in the night, like bats. Maybe Door makes him up with a beep beep and the air changes. I think Ma doesn’t like to talk about him in case he gets realer.

Through Jack’s innocent observations, the reader learns of his horrifying captivity. The fact that the innocent narrator does not understand his own predicament enhances the tragic sadness of his situation. In every respect, Donoghue’s choice to reveal the story from Jack’s point of view makes Room memorable and creative. I hope this helps!