Canada's involvement in the Revolutionary War comes up often enough on this sub that it is addressed by several sources in the FAQ, one of which is a previous answer of mine. I am copy-pasting my previous response here with some minor changes for clarification:

Colonies in Newfoundland and Nova Scotia were founded by the English in the 1600s primarily as seasonal fishing colonies. Fishermen from England and Scotland would come over for several months each year to fish and then return to the British Isles afterward. The climate in Newfoundland and Nova Scotia was not entirely hospitable for agriculture, so settlers who came to stay in America permanently tended to go to one of the "Thirteen Colonies" to the south.

But Newfoundland, Nova Scotia, and (what are now) the other Maritime provinces were in dispute between England and France. France had settlements on these islands as well, administered by them as the colony of "Acadia". So it wasn't only climate that discouraged settlement, it was the precarious political situation there, too, that made the Thirteen Colonies to the south a more attractive destination to English/British colonists.

The rest of present day Canada was firmly in control of the French and founded as New France in the 1600s. The British didn't take possession of what are now Prince Edward Island, New Brunswick, Quebec, Ontario, and all lands west until after the outbreak of the French and Indian War in 1754, and of course, that possession wasn't recognized by France until the Treaty of Paris of 1763 that brought the French and Indian War to its conclusion.

At the end of the war, the Acadian population was largely deported, mostly to either Quebec, Louisiana, or one of the French colonies in the Caribbean. To aid in maintaining British control of the land and keep the French (and their allied First Nations) out, the British government offered free land in the Maritimes to settlers from the Thirteen Colonies willing to relocate. About 8000 American transplants responded to the call, and these transplants made up the majority of the English speaking population of Canada before and during the Revolutionary War.

So at the outset of the Revolution, "Canada" as we know it today was primarily made up of the small English town of Halifax (founded in 1749) and English-speaking farmers in the surrounding countryside, as well as the French towns of Quebec and Montreal and the farmers in the surrounding countryside there along the St. Lawrence River.

It was at this time that the seeds of unrest began to blossom in the Thirteen Colonies against British Parliament. The French-speaking Canadiens also distrusted the British government who they now found themselves ruled by. They were particularly angered because the Brits were essentially excluding Catholics from serving in local government offices by requiring an oath of office that Catholics would be unwilling to make. And being French, the vast majority of the population was Catholic.

In an effort to make sure that the political unrest in the Thirteen Colonies didn't spread to Canada, the British Parliament passed the Quebec Act in 1774, which not only allowed the Canadiens to freely practice Catholicism, even among office-holders, but it also expanded the colony's borders to include the land south of the Great Lakes and west of the Appalachians ("Ohio Country", now the states of Michigan, Ohio, Kentucky, Indiana, Illinois, and Wisconsin). This law actually had several purposes: it kept the French Catholics from joining the rebellion, while it also demonstrated to the Thirteen Colonies that if they did not cooperate with Parliament, they would not be treated well, and if they did cooperate, the would be treated well.

The Quebec Act placated Quebec but roused the Thirteen Colonies, Virginia in particular since they had claimed "Ohio Country" since before the French and Indian War began (and a dispute over its possession had largely started that war). Thus, the Quebec Act was cited among the "Intolerable Acts" that were seen by the Continental Congress as justification for rebelling, and was mentioned in the Declaration of Independence in the twentieth grievance ("For abolishing the free System of English Laws in a neighbouring Province, establishing therein an Arbitrary government, and enlarging its Boundaries so as to render it at once an example and fit instrument for introducing the same absolute rule into these Colonies").

The Continental Congress did send letters to the local governments of all the Canadian colonies inviting them to join the Congress and, thus, the Revolution, but they did not receive a favorable response. However, Quebec mustered together two regiments of Patriots who fought alongside Washington's Continental Army.

The Continental Army tried to invade Quebec in 1775, led by Benedict Arnold before he switched sides, but this invasion was unsuccessful. Likewise, Massachusetts militia men attacked Nova Scotia at the Battle of Fort Cumberland in 1776, and were joined by some pro-Patriot Nova Scotians, but were also unsuccessful. As one might imagine, the population of Nova Scotia was largely Loyalist. After all, these were mostly people who had answered the British government's call for settlers little more than a decade earlier, many coming after the political turmoil had already begun. They weren't going to risk giving up the land they'd just been granted by the British government, and had relocated to Nova Scotia to claim, only to lose it in an unsuccessful revolution. At the end of the Battle of Fort Cumberland, many of the most fervent local Patriots were forced out of Nova Scotia, and the colony remained firmly loyal to Britain for the remainder of the war.

Officially, though, Nova Scotia remained "neutral", but the British Navy launched many of their expeditions from Halifax, so it maintained a British military presence for much of the war.

Thus, Quebec didn't rebel during the American Revolution because the British government had essentially ceded all the major political issues there to the local population. And the colony of Nova Scotia didn't rebel due to a small, recently resettled population pre-disposed to Loyalist sympathies, who kicked out many of their dissenters early on in the war. Same deal with Prince Edward Island, whose population was even smaller. The rest of Canada at that time fell under the administration of one of those three colonies.

It should be pointed out that there actually was a rebellion in Canada many years later, in 1837-38. These were separate, but related, rebellions in "Upper Canada" (Ontario) and "Lower Canada" (Quebec) both of which were unsuccessful but did result in expanding local governance in Canada that paved the way for self rule in 1867. But that was all many years later, and after the United States and British Canada had fought the War of 1812.

SOURCES:

The Province of Quebec and the Early American Revolution by Victor Coffin

Military Operations in Eastern Maine and Nova Scotia During the Revolution by Frederic Kidder

Benedict Arnold's Army: The 1775 American Invasion of Canada During the Revolutionary War by Arthur Lefkowitz

A History of Nova-Scotia, or Acadie by Beamish Murdoch

The War that Made America: A Short History of the French and Indian War by Fred Anderson

Further Reading:

The Battle for the Fourteenth Colony: America's War of Liberation in Canada by Mark R. Anderson

Our Struggle for the Fourteenth Colony by Justin Harvey Smith

United Empire Loyalists: A Chronicle of the Great Migration by William Stewart Wallace

The Relations of the United States to the Canadian Rebellion of 1837-1838 by Orrin Edward Tiffany