Foot in the door method is the tactic used by a person to make other person agree on the bigger request by first proposing a smaller request.The larger request that is asked afterwards is the actual desired intent of the requester .
Smaller or initial request is just made to achieve acceptance so that at next step bigger request can get a way to receive acceptance as well.This makes it easy to receive approval on intended wish.
Because after world war 2 America needed to heal and regrow crops and regain money and that takes a while so people moved to Canada so they wouldn't have to wait.