When Mary Shelley wrote the book "Frankenstein" she wanted the reader to be able to become the person that was sitting down listening to the story as it was being told to them by the captain of the ship. One may recall the story of "Frankenstein" began as a ghost story told while spending a time with friends and exchanging ghost stories. She wanted to capture the effect of the reader who has no knowledge of the events to be told them through the captain.
Shelley also wanted the reader to step out of the captain's world and into Victor's world. By having Victor become the narrator the reader is able to feel his anguish at his failure to create the perfect man and the duress he undergoes after he made his creation. The theme in Victor's setting is man versus self-created monster or man versus self (since it was Victor's own greed that led him to create the being from convict parts, etc.).
Shelley switches back to the narrator because it is the idea that the monster has some degree of humanity present. Victor can not be the one to share this because he only sees his failure and horror in the creation. A neutral party such as the captain can best share the monster's own conflict and need as he witnesses his father'/creator's death.