I'm drawing primarily from John Brebner's The Neutral Yankees of Nova Scotia.

There were a number of factors. First Nova Scotia was relatively unpopulated. It only had a population of roughly 20,000. Most of the "Thirteen" were over 100,000 each. Only Georgia was as lightly populated, with 23,000. And Nova Scotia's population was isolated in individual villages well separated from one another. Inland travel was difficult, so travel was mostly via ship, mostly on ships from Halifax and smaller fishing craft. The rest of the population was in Halifax. Such a population had little chance to communicate with one another. There was a Nova Scotia assembly. But it was dominated by Halifax and under close observation by the governor, and his troops. It wasn't a very friendly place to discuss revolution. Smaller town meetings and the like were banned in Nova Scotia. So the smaller towns which had some revolutionary inclinations, didn't have much chance to express them.

The people of Halifax itself were largely against the war. The merchants were making real money. And every time the British did something to cities like Boston or New York, the Halifax economy improved. Halifax saw itself as a rival to other North American towns, especially Boston. So even though about half the colony had roots in New England, it wasn't terribly friendly toward it. Halifax almost reflexively disagreed with Boston over everything. And with the war on, disagreeing with Boston became immensely profitable.

On top of that, or maybe because of that, Halifax was the primary military base and supply depot for he war. Divided and uncoordinated, any Nova Scotian rebellion would have had to immediately deal with the largest British garrison in North America. A fact that potential revolutionionaries were all too aware of, dampening revolutionary zeal.

In addition the preacher Henry Alline had become tremendously popular within the colony. He was viewed with an almost rockstar awe. And he was utterly opposed to the war. His pamphlets and sermons were widely reprinted and read within the colony. He and other Nova Scotian Great Awakening preachers, taught pacifism and spirituality as the solution to the difficult times. And the locals ate it up.

In all it added up to a population that really wasn't eager to follow the call to revolution, one that profited off neutrality, and one that wasn't able to really do anything like raise troops or assign delegates to congress, even if they had wanted to.

I'll repost a slightly reworked version of my first answer with a followup in reply to it:

Firstly I should establish that at this point Nova Scotia encompassed New Brunswick as well. New Brunswick and Cape Breton were created after the war to divide NS in case of revolutionary unrest. PEI was just being settled and was so sparsely populated that Revolution was not really in the cards for the island, although it was raided by American privateers during the Revolution, just as NS was. As such the question "Why didn't the Canadian colonies join the American Revolution?" can be safely divided into two: that of why Quebec didn't join, and that of why Nova Scotia didn't join. I would be interested in any account of Newfoundland during this period but have not encountered anything more than the odd footnote on the topic of Newfoundland and the Revolution. I'll leave the question of Quebec to one better versed in its history and stick to trying to answer the Nova Scotia question.

I peg the beginning of the American Revolutionary Crisis as being 1765, with the passage of the Stamp Act that did so much to enrage and alienate North American newspaper printers and more importantly their subscribers. Sloan and Williams write in The American Revolution and the Press that this unified mass media opposition to the Stamp Act was unprecedented in history, and the perception of unified outrage that it created was a huge factor behind getting different British North American colonies to unify in opposition to Parliament's taxes.

During 1760's French power was shattered in Eastern Canada and their people were deported en masse to Louisiana. To fill the vacant French Acadian lands Governor Lawrence issued a proclamation promising free land to any New Englanders that moved to Nova Scotia. Furthermore in 1758 he promised to form a Legislative Assembly, with two members elected from every township of fifty families or more, in order to attract the New Englanders who were used to their own democratic, decentralized form of township government.

Before this NS had been ruled by a central, appointed Executive Council that served at the pleasure of the royally appointed governor. Giving townships representation and some modicum of authority was an about-face in Imperial policy, which had previously been against developing Nova Scotia into a "New New England", as the Imperial government found the decentralized New England system irritating, as local township governments possessed the authority to impede the efforts of London authorities to impose economic and political agendas on the colonies.

My favourite book on NS during the Revolution is Fault Lines of Empire by Elizabeth Mancke, and she singles out township government as the point of divergence between New England and NS that sees the first revolt and the other remain neutral. She convincingly argues that Lawrence's system only appears to be similar to the decentralized New England system, but that the Executive Council actually still maintained sufficient control to stymie local government in NS, in large part due to the Executive's ability to use discretion in granting townships, and that it is this more than anything that kept NS neutral. It is a great piece of historical work, very focused and extremely methodically researched.

But the PR move of establishing township governments worked, despite their inherent weakness, and New Englanders flocked to NS in search of homesteads. The upshot of this is that by the time the American Crisis began over half of the population of NS was "American" in origin, that is they had immigrated from New England under the terms of Lawrence's Proclamation, bringing their culture, political views, and contacts/family connections to New England with them. This raises the question: With New England being a hotbed for revolutionary sentiment, and with New Englanders having a strong tradition of political participation, organization, and petitioning through their strong township system, why didn't Nova Scotia organize and revolt as the other colonies did?