The FAQ has several links to previous times this kind of question has been asked. However it is always an interesting question to explore.

One of the common points in those past responses is the Quebec Act, which guaranteed the (primarily French and Catholic) colony of Quebec certain rights - primarily the free practice of Roman Catholicism and use of French Civil Law, as well as expanding Quebec's territory. As a result of this situation, there was little popular support for joining the Revolution and putting those new-found freedoms at risk.

The other colonies didn't participate for other reasons that those responses linked from the FAQ touch on as well - related to matters of population, geography, culture, demographics, and administration.

They were in fact invited to be the 14th colony as you can read in Letter to the Inhabitants of the Province of Quebec But as /u/Master_Gunner said the Quebec act was pretty favorable to the French Canadian population compared to the anti-Catholics in Boston. The lack of responses and the overall context led to an American invasion of the province of Quebec. The American occupied forts in the valley of the Richelieu and Montréal in 1775 but they were forced back at Quebec City.

Nova Scotia is an interesting case of a modern Canadian province which nearly joined the Continental Congress.

According to "Nova Scotia During The American Revolution" (Bill J. Wilms) the 200,000 inhabitants of Nova Scotia had strong ties to New England. In fact, many people were from New England.

Just as tensions were boiling over and the revolution was about to break out in earnest, Nova Scotia went through "The Great Awakening " which was a deeply spiritual Catholic revival. This shifted the culture significantly away from the fervently anti-papist New England states.

In addition to a Catholic cultural awakening, the British had a massive fleet and garrison at Halifax. As the war progressed this city became the main transit hub for British military en route to American battlefields. This meant that any serious revolt in the colony would have been quickly crushed by overwhelming British force. The bulk of the Royal Navy was stationed here as a bulwark against any French attempt to use the revolution as a pretext to retake their lost empire.

Nevertheless a small group of Nova Scotian revolutionaries did attempt to overthrow the British in the colony and there were several small-scale "battles" (really skirmishes) at Fort Cumberland, Miramichi and a siege of the town of St John (in today's New Brunswick, which was Nova Scotia at the time). A significant number of Nova Scotia men served in the Continental Army and a delegation sat on the Continental Congress representing the "14th Colony", although this delegation did not have any real authority to represent the people of Nova Scotia who, by and large, were uninterested in joining any United States scheme.

As others have said and I will further elaborate, Canada wasn't equally British in the 18th century. It was a recently conquered formerly French colony except in the Maritime Provinces, in fact the war that conquered it helped lead directly to the American Revolution, as did the Quebec Act giving it significant autonomy. Canada became British in nature after the American Revolution caused Loyalists to flee to Canada and British colonists to seek landholdings in Canada.

In fact Canada, in this usage only meaning the modern provinces of Ontario and Quebec, only lost its French majority in the 1830's and 1840's. It wasn't British at the time, and the French were mostly happy with the status quo, particularly since joining America would erase the privileges the British had given them.