It's because Canadians are in love with the idea that if we all just join hands and sing "Kumbaya", all will be well. To most Canadians, the military is seen as a somewhat 'icky' kind of thing, best not seen and not heard, but they want to have some way of getting Canada a place on the world stage, and peacekeeping seems to be the way to do it. Canadians seem to be most comfortable with the idea of soldiers as helpful Boy Scouts, and not as men armed with rifles, bayonets and other weapon systems who are trained to kill other people when the situation demands it.
I think the near-totally pacifist stance has a lot to do with the tremendous losses of people Canada sustained in the First and Second World wars - that is, so many people lost family members in those wars and they would like it very much if that kind of thing was never repeated. That's an admirable desire and goal, but it ignores the fact that the world has been exposed to conflict and wars since day one, and there doesn't seem to be much evidence that that natural state is going to change for the better anytime soon. By not having at least minimally competent military forces, you actually increase the risk of people dying or being injured.
I also think another part of it comes from having elected so many French-Canadian prime ministers who, because they came from Quebec, tended to have a mindset that favoured a pacifist outlook.
Just my two cents - or maybe $0.01, adjusted for inflation.