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It's all there in the FAQ.
The answer is, the area that is now Ontario was basically uninhabited and Quebec did not see themselves as British, they saw themselves as French.
New France had just be conquered by Britain in the Seven Years war and their rule turned out to be reasonably tolerable. They were allowed to continue practising Catholicism and French civil law, all while getting new access to the British trade network. They didn't want to count on the Americans extending the same privileges just so they could be occupied by a different army of protestant anglophones.
There wasn't many anglophones north of the Great Lakes until loyalists settled there specifically because they opposed the war and they were fleeing the United States.
The British colonies which now make up Canada, consisted at the time of the Revolution of:
Canada (as differentiated from Nova Scotia, Prince Edward Island, Newfoundland and Labrador, which were different colonies) which had been French until 1763, only 13 years before the Revolution.
Nova Scotia, which included New Brunswick, which was populated roughly 1/4, 1/4, 1/4, 1/4; by New Englanders (who had taken over land from the expelled Acadians), Acadians (French who had evaded expulsion, or returned), British settlers who had recently arrived (mostly around the naval base of Halifax), and Native Americans (mostly Micmacs) who had been there longer.
Prince Edward Island, which was inhabited by Acadians and English settlers.
Newfoundland (and Labrador) which was inhabited by English settlers.
The American Revolutionaries hoped to attract (or compel) at least Canada and Nova Scotia (Nova Scotia included New Brunswick until 1784, after the revolution) to join the rebellion.
There were Revolutionary sympathizers in Nova Scotia. The British had exiled many of the French Acadians from Nova Scotia during the French and Indian War. (many of these eventually ended up in Louisiana where they were known as "Cajuns" - short for "Acadians".). To replace the exiled Acadians, the Empire recruited settlers from England, and from New England. At the time of the revolution, Nova Scotian population was about one quarter remaining Acadians, one quarter MicMac Native Americans, one quarter recently arrived from England, and one quarter recently arrived from New England.
Some Nova Scotians (particularly of New England derivation) sided with the Revolution, and with some American military help tried to seize the strategic Fort Cumberland (dominating the narrows between peninsular Nova Scotia and what is now New Brunswick). This attack and siege was narrowly beaten off when re-inforcements arrived by sea from Halfax. It was the strong British fleet and military base in Halifax, together with a sizeable population recently arrived from Britain who had no rebellious inclinations which kept Nova Scotia loyal.
The Rebels also had high hopes of support from Canada. Britain, however, had been particularly accommodating to the French Canadians, and buttered them up, so they were less supportive than the Rebels hoped. After early Rebel attacks on Montreal and Quebec were unsuccessful, Canada was secured by the Empire.
Besides remaining loyal to Britain during the American Revolution, the other main thing that set Canada on a different path to the United States was the immediate post revolution immigration of 40,000 Empire Loyalists. After the Empire lost the 13 American colonies, some 70,000 loyalists or "Tories" left the United States to remain in the British Empire. The 40,000 who went to Canada settled in Ontario, Quebec, and Nova Scotia, and provided a boost to the population of new settlers opposed to the United States, who would prove valuable in fighting for and protecting Canada in the war of 1812.
(source: http://en.wikipedia.org/wiki/United_empire_loyalist)
Jonathan Eddy was the main supporter of the American Revolutionary cause in Nova Scotia. He was a New England planter from Massachusetts who had settled in Nova Scotia after the Acadians had been expelled. He was an elected member of the General Assembly of Nova Scotia, and a strong supporter of the Revolution. He led the attack on Fort Cumberland.