1640 is not really such an arbitrary date as it might appear at first. There is a webinar on YouTube about researching 17th-C New England Ancestors, where an expert on New England genealogical research explains the way historic events in England led fo Puritans first fleeing to New England because of severe persecution from the official state church (the Anglican church headed by Archbishop Laud and ultimately by the King), and then taking up arms and actually overthrowing (and executing) the KIng and forming a whole new government, headed by a Puritan, Cromwell, as Lord Protector.
The persecution reached its zenith during the 1630s and the armed revolt started about 1640.
Mr. Dearborn displayed a graph showing the number of people who came to New England during each year of the 1600s, from which it was easy to see how fast immigration took off in 1633 when the worst of the perseuction started, and then, how it suddenly plummeted to a mere trickle in 1640. You could really visualize how the people who had been coming to New England suddenly decided to stay and fight things out at home. So,1640 really does mark the end of the historic event which has come to be described as the Puritan Great Migration.