To answer this question we need to carefully consider the purpose of William Bradford's History of Plymouth Plantation. This book is generally thought of as a history of life in early colonial New England. Because it is the only work of it's kind to address this time and place, it is usually studied in school as an example of colonial literature and an important historical document. However, we need to keep in mind that students, especially high school students, are typically exposed to only a small portion of the book.
The oft-used high school textbook Elements of Literature—Fifth Course has only excerpts from Chapters 9 and 11 of the book. Entirely omitted is the story of the Puritans in Holland and England prior to their journey to America. This part of the book, which covers about 14 years of Puritan history, details their difficulties in finding a place to worship. By the time the finally reach the New England shore in September, 1620, they have already faced many hardships and deprivations.
Bradford's purpose is not so much to illustrate an important aspect of America's origin and development, but to show how the Puritans are working in accordance with God's will to establish a Godly settlement in which they can live according to their own beliefs.
We see this religious emphasis in Bradford's interpretation of events. When relating the fate of a particularly unlikable non-Puritan sailor, Bradford writes:
But it pleased God before they came half seas over, to smite this young man with a grievous disease, of which he died in a desperate manner.
Bradford attributes the death of the sailor to God's vengeance. In fact, just about everything is attributed to God's will in one way or another. It is entirely natural for Bradford and the Puritans to look at life this way, so we see quite a few similar instances throughout History of Plymouth Plantation. Bradford wants to show the that the Plymouth colony has successfully fulfilled God's plan.