Well, as a Canadian, I can tell you a large part of it is because the first 'Canadians' came here from the UK. A lot of how we pronounce words and how we speak is due to our British blood line. Also, not all Canadians say 'Aboot' or 'Eh'. That is like people assuming everyone in the US speaks like a Hill Billy. We have select provinces that do use the staritypical accent that many countries view as the 'Canadian Accent'.
I am mostly Scottish, so even though I was born and raised in Canada, I do have a slight Scottish accent. Another part of it all is that Canada is so open to other cultures and languages, so we all just tend to mingle and mix out accents lol