Why didn't Canada join the Revolution?

Because a lot of things that pissed off the proto-rebels were for the benefit of the colonies Great Britain had recently taken from France, and there wasn't any sense of community between the two groups.

Quebec was largely French, and what wasn't French had come from GB recently, which was very much not the case with Massachusetts et al. Thus, there was little cultural connection, and much less discontent with the tyrannical colonial overlords, since GB was also a lot nicer to Quebec in terms of actual representative government.

The Quebec Act of 1774 pretty much solidified the divide; the British and French legal systems were given equal weight in Quebec, tithes to the Catholic Church became mandatory, and the province expanded east to Labrador and west to the Ohio and Mississippi rivers. The last part was the worst, since it meant the thirteen colonies couldn't settle there themselves. Quebeckers very much liked this, but Ben Franklin organized the first Continental Congress largely in response to the Intolerable Acts, of which this was the fifth and last.

Relations between Quebec and the United Colonies went downhill from there, and in 1775 American armies invaded Quebec, taking Montreal but being halted at Quebec City; once the British navy came in the spring, they fell back. In Nova Scotia, privateers raided small outposts, and any chance of bringing the Canadian colonies into the American fold disintegrated with the New England sympathies burned and robbed.

Once the war began, Canadian settlers were also the sole traders with England in the New World, which boosted their economy nicely, and British forces ensured the safety of fur transportation. Plenty of pro-British colonists in the rebellious provinces moved to Canada, moving the popular sentiments of both regions further apart.

So, Canadians didn't join the Revolutionary War because they didn't have good reason to do so, and plenty of reason to stay with the Crown. This isn't to say that the people of the former French colonies particularly liked Great Britain, but the Americans were a lot less appealing, especially once the raiding and invading started. The Continental Congress mostly liked Canada okay, despite the religious and cultural divide, inviting the province to join in the Articles of Confederation (which accepted Canada as a member if it ratified the Articles), which never happened; there were no such special accommodations in the Constitution.