Thomas Rogers writes for BBC.com:
The primary reason for Canadians’ hard-to-identify accent is, of course, historical. Canadian English was partly shaped by early immigrants from the UK and Ireland, but it was affected much more by the arrival of about 45,000 loyalists to the British crown during the American Revolutionary War. By the outbreak of the War of 1812 a few decades later, a significant part of the population of Ontario – which had about 100,000 inhabitants – were of US extraction. The result, especially west of Quebec, was an accent lightly shaped by British English, but much more so by 18th Century colonial American English. Since Ontarians were largely responsible for settling Western Canada in the following decades, their Americanised accent spread across the country and eventually became the de facto accent for the majority of Canadians.
There are a few exceptions: Newfoundland’s population was disproportionally shaped by immigrants from southwestern England and southeastern Ireland, and since the island is geographically isolated and only joined Canada in 1949, it has retained a way of speaking that is dramatically unlike the rest of the country. Other parts of Eastern Canada – including Cape Breton Island and in parts of Prince Edward Island – have also retained distinctive accents.
But if a person travels into western Canada – into Manitoba, Saskatchewan, Alberta and British Columbia – the accent becomes more American-sounding and locally indistinguishable. “The West was a melting pot for settlers from many regions, so there was this leveling out of differences, not only in Canada, but in the US,” says Boberg. “The settlement depth in Western Canada goes into the 1890s, so it is about a century old. The time depth and the population density being very sparse, those encourage widespread leveling and homogeneity.” This makes it very difficult, even for Canadians, to tell the difference between someone from Winnipeg and someone from Vancouver – or from Seattle.
There’s an interesting change happening, which I hadn’t been aware of:
In recent years, academics have noticed that the Canadian accent is undergoing a curious change. Known as ‘the Canadian vowel shift’, the vowels among a wide variety of Canadian demographics are becoming higher and pronounced further back in the mouth – bagel is turning into ‘bahgel’, shoes into ‘shahs’. The reasons aren’t clear, and despite the name of the shift, Canadians are not alone in undergoing the change: something similar is happening in parts of the US, including California (some people are describing this process as the ‘Valley Girl-isation’ of the Canadian accent). Other parts of the US, including some northern regions, aren’t experiencing the shift, so it may end up amplifying differences between the two accents rather than muddling them.