Canadian spelling: why?
I've done nothing but sit on my rear all day trying to find you an appropriate answer. I've only come across one article online that seems to collectively dictate anything and everything that I've being reading. It seems that Canada defines the majority of its culture upon its language (and spelling).
While I have to agree with Robin Michael that you'll never really find yourself with a simple answer, I do hope this article may shed some more light.
According to him, the Canadian language, the culture, is being slowly diminished by your friendly neighbors to the south of you. (Hello!)
The Canadian culture was to be unique and different but outside factors have created mass confusion on how to spell.
(There were many other websites that mentioned bits and pieces of what this article says. I chose to link you this certain article, alone, because it was the only article to contain a bit more information closer to what you're asking.)
I posted this too soon it seems. I found this article that seems to have a somewhat similar take to the other article, but with a semi-different spin.
Taking your point to heart, I decided to refine my search, looking strictly for only books. Finding this book in particular, the Google Preview looked promising. There is a lot to read, however. I don't think you'll mind that though; you seem to really want this answer!
Soft edit: I normally dislike Wikipedia, and I don't know if you saw this or not, but I think it gives a general clue as to how Canadian English became mixed. Link
I do not think that this question is capable of a simple answer. I think that this question is more the beginning of a discussion of what it means to be a Canadian.
You could have a similar question about any variety of English.
New Zealand spelling: why?
The first time I came to this site was when I was trying to spell the word 'vacuum'. For some reason I thought it was spelt 'vacumn'. At the end of the day, the correct spelling is just a convention.
You could easily reword your question: Why do Canadians have to be different?
You might find the answer in this book by John Kenneth Galbraith: The non-potable Scotch: A memoir on the clansmen in Canada [Paperback]