I don't know.
Then again, if the criteria were simply the size of a country, Australia should be it's own civilisation too. (Funnily enough I think it was in at least one expansion. I shudder in horror just remembering it though, because they had the inanity to make the leader John Howard. That's some kind of sick joke right?)
Anyway, yeah, sorry to say Canada just isn't very memorable as a civilisation. Seeing America represented in 4000BC is always a bit silly, but America dominates the modern world for some reason.
England was a major empire for over a century.
China is on it's way back to being a significant force, but also was the single most advanced civilisation on the planet for near to a millenia.
Egypt is now an insignificant country, but was historically very prominent.
Greek culture dominated much of the world, even as the Roman empire took over most of ancient Europe.
Spain essentially took over most of south America, and it's influence in that regard can still be felt, even if it has no empire to speak of today.
The Dutch held an immense amount of influence over trade in Europe, and owned indonesia and several other colonies (as well as New York formerly being New Amsterdam). For one of the tiniest countries in Europe it's influence is quite dramatic.
Germany tried to take over large parts of Europe. at least twice. Not to mention their history in Europe generally.
France, aside from having a long history also tried to take over.
Russia has a lot of history before that, but there is the whole USSR thing to think about too.
Japan is a small, frequently xenophobic country, historically, but they've had frequent conflicts with china, and again they've tried to take over Asia.
India may have been an English colony at some point, and to the modern world looks relatively primitive, but it's history further back is very impressive, and quite a few things have their origins in India.
The Aztec, Incan and Maian civilisations were also nothing to be sneezed at, even if they all seem to be gone now.
Compared to that, I can't really think of anything significant about Canada. That's not to say there's no reason to include it as a civilisation, but there are about 200 countries on the planet right now, many of which have as much claim to being included as Canada does.
- It does surprise me they didn't at least include it as a nation state in Civ 5 though.
(Though going through what I know about the civilisation games I'm struggling to find any viking inspired civs. You'd think that would be significant right?)
Still, at the end of the day it's just a game. The reasons the Devs chose whatever civilisations are in the game can be fairly arbitrary.