Before anyone puts down Canadian English as non-existent and/or irrelevant, I would like to point out that the "Standard Canadian English" dialect - e.g. the kind of English spoken in Ontario (excluding the Ottawa Valley) and all points West - comprises the largest area in the world in which a single dialect of any language is spoken.
Whereas you can't go more than a few dozen or hundred kilometres in the US (or any other country) before the accent and word usage changes noticeably, Western and Central Canada comprises a nearly 100% uniform dialect over thousands of kilometres. It's essentially impossible to tell if someone is from Ontario, Manitoba, Alberta or British Columbia from his or her voice (even to a native) - whereas it's relatively easy to tell if someone is from New York, Kentucky, Michigan, Illinois, North Dakota or California.
As for why all Canadians West of Ontario speak the same way, I think it has to do with when the country was settled.
The US West was settled earlier than the Canadian west, and largely by English speaking Easterners (who already had all sorts of regional accents). These people had more time to develop their own local accents and usages and wouldn't have had much contact with outsiders.
The Canadian West, OTOH, was settled later by Ontarians and mostly non-English speaking immigrants. These people (or, more to the point, their children) would have been learning to speak the language just as radio, telecommunications, and (later) television was coming of age. The voices they would have heard on these new mediums would have been Ontarians. Hence, Westerners learned to Speak like Ontarians because it's what they've always heard and they didn't have the time in isolation to develop their own accents.
Source: http://www3.bc.sympatico.ca/linguisticsissues/britishcanadianamericanvocabcanadianpron.html | 
