London may have been naive in transferring to the Province of Quebec lands west of the 13 colonies, as well as guaranteeing the conquered inhabitants their language, religion and legal system, not to mention sparing them from deportation; this was unprecedented in British history, and even the Scots were forbidden to speak Gallic and wear tartans after the defeat of the Jacobite rebellion. The Americans were infuriated that they did not reap the windfall they expected after having participated in most of the battles against the French up to 1759. But the main reason for the American revolution was the high taxes levied on the American colonies by London in order to pay the huge debts incurred during the 7 years war against France.
In any case, the Act of Quebec was proclaimed in 1774, and the American revolutionary attack on the Province of Quebec took place in 1775. No matter the reason, Sir Guy Carleton, the Governor of British North America, had barely escaped being captured in Montreal by Montgomery, and upon his retreat to Quebec City, realized that the garrison of British regulars there was too small to prevent Quebec and then the rest of British North America from falling to the Americans. He had to reactivate the French Canadian militia. He gave every able bodied male within the ramparts of Quebec 4 days to decide whether to leave or to join the militia. 543 enrolled in the Canadien led militia and few left. Along with fighting for the Crown came more rights, culminating in the Constitution Act of 1791 that created Upper and Lower Canada, including their legislative assemblies.
Despite the American fury at the unprecedented largesse granted by the British to the French Canadians, the Americans made frequent overtures to the Canadien to join them in overturning British 'oppression', from 1775, to the war of 1812, to Louis-Joseph Papineau during his exile in the US after the rebellion of 1837, and even during the Fenian raids. But they stuck with the Crown.
Without the French Canadian militia, Quebec and British North America would have fallen to the Americans in 1775.
The popular notion amongst Canadians is that the 'French' should have been assimilated or deported after the Plains of Abraham, but should basically keep their mouths shut and this bilingualism nonsense should end. But Quebec is French speaking and Canada bilingual because the British needed them at a critical moment in history. The same happened again in 1813 at the battle of Chateauguay.