Why was Canada not a part of the United States, and when was the United States founded?

At the time of the American Revolution in 1776, Canada was only a loosely-owned British colony with a high density of French inhabitants. The boundaries that represent modern-day Canada were not part of the political map of the late-eighteenth century British colonies. The enormous territory west of Ontario, northwest through the Yukon territory and into Alaska, had been almost entirely unexplored. The primary areas of settlement of the French, British colonists, and British troops were in the walled-city of Quebec and along the coasts of Newfoundland. Still, this settlement was sparse, and the majority of the British colonial population resided further south, in New England and the Middle Colonies.
The primary reason that the British-owned regions of Quebec and Newfoundland did not become a part of the new American Union is simply because this territory was not a part of the British concession granted to America in the Treaty of Paris, signed 1783. The British did, however, grant Americans exclusive fishing rights along the abundant coasts of Newfoundland and along the St. Lawrence River, and they also allowed them to dry their catch along the unsettled eastern coasts of Canada.
By and large, because of the minimal role the Canadian colonists played in the Revolutionary War (the only main battle being an abortive attempt to take Quebec from the British in 1775, which was cut short because of a smallpox epidemic) and because much of the Canadian hinterlands fell well outside of the limits of the colonists's agricultural and shipping interest further south, the new Republic did not press demands for the integration of Britain’s Canadian territory.