(The premise of your question holds up to 1931, at least. Post-WWII and the independence of British and French territories, it stops.)

In part because Canada was where the British loyalists were expelled to in the US War of Independence; and the population of Canada was always pretty pro-British anyway.

Canada is the parts of North America which the US did not care to grab (post- War of 1812, when Canada established it would defend its borders, the US did not care to bother Canada - there was too much bang for the buck in stealing Spain's Caribbean empire and the Philippines, and then "democratizing" central America). It was more pragmatic for the US to expel the loyalists to a territory they didn't care to fight over (in the 18th century), than to claim all of North America and have to fight them in a war the US didn't want and couldn't afford.

The 1867 British North America Act established Canada as a separate dominion, and there wasn't ever a strong political movement for independence.

A question of this sort is going to generate a lot of opinion, so there may not be a definitive answer or you'll have a lot to sift through.

I expect Canada didn't "emerge as a power" because Canada didn't want to.

The Game of Empires

Spain started grabbing land in the New World in the late 1400s and 1500s. The Portuguese similarly started grabbing land in the Indian Ocean in the 1500s. The other sea-going European powers (particularly Britain, France, and the Netherlands) also started grabbing land around the world. Italy and Germany didn't unite until the late 19th century. The Austro-Hungarian, Ottoman, and Russian Empires were pretty contiguous and had their own limitations to expansion.

The United States didn't really emerge as a power until it started realizing imperial ambitions in the late 1800s. Yes, the U.S. pushed westward, taking and buying land from Spain, France, and Britain, and this was pushed by public opinion (see "Manifest Destiny"). By the time the U.S. was contiguous from Atlantic to Pacific, public opinion was primarily isolationist; the politicians that wanted more land had to spin the truth so the people might accept that it wasn't a land grab.

Similarly, when it formed in 1871 in the wake of (or as a result of) war with one of the Great Powers, the German Empire realized it was late coming to the Game of Empires. Fueled by public opinion (or, at least, several of Germany's leaders), German still reached out and grabbed some land in Africa and some islands in the Pacific.

By the time Canada became independent and was able to dictate its own foreign policy, World War I had ended with horrific consequences for the European players in the Game of Empires. (The U.S. and Japan weren't as badly affected.) I expect that, in reflecting on World War I, Canada didn't want to join that particular game.

The Game of Money

While the land area of Canada is slightly larger than that of the United States, its location further north has prevented Canada having a population to compare with the United States. Most, if not all, of the major population centers in Canada are in the south, often within 100 miles of the border with the United States. Many of the exceptions would be on the Atlantic Coast, where the Gulf Stream makes for milder climates.

Canada has plenty of natural resources. Canada also has the infrastructure and technology to compete with the other nations on the financial market.

Canada's location, however, makes it more difficult to extract those resources. It's not impossible, of course, just more difficult. The history of rail building in Canada has a couple of particular dark periods for the government economically. Rail is one of the most efficient means to get natural resources to refineries.

Again, Canada probably could become a bigger player on the financial market if the right people tried hard enough.

Canada is part of the G-7. (Along with Britain, France, Germany, Italy, Japan and the U.S.) In this regard, Canada IS a world power.

But not all world powers are in the G7. (Russia, China and India come to mind).

When do you mean?

Up until 1867 it was British - and you could make a reasonable argument that geopoliticaly it was pretty much British until after WWII. Really, the Suez crisis marks the beginning of a separate Canadian foreign policy.

In the cold war it was/is a strong US/Nato ally. It's not really big enough in population terms or political will to be a major player and it's location is too strategically important to remain neutral.

Post cold war, it shifts with the politics of it's ruling party and the US presidency.

Why it didn't throw off the imperial yoke in the 18/19C and become a global superpower to challenge Britain and the emerging US? Well that would have been rude.

Until very recently Canada was a much smaller (population-wise) concern than the UK or France. Today those two countries are in the top 25 in population (65 mil and 62 mil), whereas Canada only has 34 million, about the same as Iraq. This is actually a pretty good relative improvement for Canada though. They didn't even break 15 million until around 1960. At that time, the UK had 52 million (and that number was much more impressive than today).

Heck, Greater New York City is only a bit smaller at roughly 22 millon.