New England resident chiming in here.
To say we joined Canada in 1780 isn't really accurate, to be pedantic, as the
Federated Kingdom of Canada didn't exist for us to join for another fifty some years. And there was a lot of pro-rebel sentiment in New England, especially as many of the formative events of the revolution happened there, and there was the sectarian dynamic between Anglicans who were generally supportive of the crown and nonconformists who were more inclined towards the rebels, and the middle and southern colonies were more Anglican than puritan New England was. The Philadelphia College historian Newton Gingrich who has written extensively on the American Rebellion talks about this in
For King Or For Country, looking at the conflict as an extension of the English Civil War and the Restoration after a century-long armistice, pointing out that it fell upon similar lines (republicans against royalists, nonconformists against Anglicans) and estimating that there were more loyalists in the middle colonies than there were in New England. In fact, while the rebels were a minority, a large chunk of the population in New England were ambivalent or just wanted to keep their heads down and didn't actively support either side, which was true in most of the colonies actually.
I imagine the real reason New England didn't fall was that it's elites realized that the Francophilic and republican impulses of the rebels and the potential of bad relations with Britain even if independence was won would've economically crippled the region which was dependent on maritime pursuits. Plus, New England juts out hundreds of miles into the North Atlantic and was close to the provinces of Quebec and Nova Scotia which the British held despite the rebel sympathies of some there and so could easily be recaptured. And even still, you had people like Daniel Shays and Ira Allen in what was then the frontier agitating against the British for years afterward and it was only after the Second Anglo-American War and the Federation that the Americans accepted that regaining New England was no longer realistic.
OOC - is Canadian New England a single Province, or is it divided into its constituent parts as in OTL's USA?
OOC: I say divided into a few provinces; they would've joined whatever federation is set up as is and likely wouldn't have merged. Of course, Massachusetts evidently retains Maine and Vermont if in British hands probably remains part of New Hampshire; IIRC the people there wanted to remain in New Hampshire but the government of the state had already agreed that the land belonged to New York which was the impetus for succession.