Because they were comparatively useless to colonizers economically.

Africa, India the Middle East, and the Central part of the Americas (ranging boradly from Northern Virginia to Southern Brazil) received more attention from European colonizers than areas of the world that lacked considerable mineral wealth or which were further from the equator, namely North America, Australia, Uruguay, Chile, Australia, and New Zealand. The motivations of European governments in the age of imperialism were largely economic, and these colonies simply had less to offer.

Economies that were based around specific resources - namely cash crops (most of the "central band" of the Americas, Egypt, the Indus Valley, and Indonesia), mineral resources (as is the case with much of Central and Southern Africa), or oil (Mesopotamia) developed highly stratified societies. An elite which controlled those resources both before and after independence sat at the top. In the Americas, this elite was almost entirely of European descent, with some exceptions such as the Duques de Moctezuma and other native royalty who imperialists considered prestigious enough to legitimize. The "establishment" in the "central band" of the Americas ran large plantations which provided cash crops to Europe, and relied extensively on forced labor from natives and African slaves. In the Middle East, India, and Africa, the establishment consisted of a hybrid of Europeans and local elites who had agreed to collaborate.

This establishment found itself in a dependent relationship with colonial powers, providing raw commodities to Europe in exchange for manufactured goods, and, in the imperial period, security. After independence, the local establishment largely remained in power, the difference being that they no longer shared administrative and military responsibilities with Euroepans. The elite quickly monopolized all business, administrative, and military functions after independence. With teh exception of Middle Eastern oil, all the valuable commodities I listed depended heavily on a cheap supply of labor. Because of this, the post-colonial elite of all commodity-based economies had little incentive to invest in education, which could potentially radicalize their workers and drive them to careers outside low-paid (or unpaid) commodity extraction. Additionally, the establishment of commodity producing countries had strong incentives to keep the popultion disenfranchised. A study by economic historians Jonathan Conning and James Robinson across nations in the Americas revealed a strong negative correlation between cash crop production on one hand and both voter enfranchisement and investment in human capital on the other. In other words, if your government consists of landlords or mine owners, it's probably going to do everything in its power to keep the rest of the population poor, uneducated, and unskilled.

This all matters for political stability, because the economic models of commodity-dependent post-colonial countries encourages inequality, rule by an elite, continued dependence on foreign powers, repression, and dictatorship. All these things increase resentment within the population and the incentive for infighting among the elite, as all of these factors make takeovers more lucrative. In an intransparent government, maintained by force, and with few decision makers, the opportunities for corruption and self-aggrandizement are far greater than in a democratic state policed by its citizens and press.

The countries you listed benefited from being comparatively useless. Colonies in the far north or far south of the world generally were not founded strictly for economic purposes, and served other purposes like geopolitcal influence or land in which to settle prisoners and religious dissidents. Because of this, they developed economies that either (1) followed a "homesteading" model of agriculture, which involved a large number of acres per farmer (therefore a higher average agricultural income) or (2) mirrored European industrial growth. Whether 1 or 2 happened dependend largely on population density, with New England and the Mid-Atlantic states achieving the greatest level of industrialization by the late 19th century.

Both these economic models were more dependent on skilled and educated labor. For this reason, societies tended to be more democratic. The benefits from investing in education were greater, as were the "costs of repression", as the elimination of skilled labor would have more detrimental effects on the economy.

As a final note, Canada and Australia had a foreign policy tied to that of the UK until World War 2, or perhaps until the Suez Crisis. If they ever had serious threats to their stability before then, the UK would have certainly intervened to help them.

In summary, while the equator was converted into an appendage of Europe, the less lucrative far north and south were made into extensions of Europe - a "place like home" for Europeans who no longer wanted to live in Europe.

Two caveats before I paint with too broad a brush - first, the Southern US is generally considered a "hybrid" economy by economic historians, which was partially reliant on plantations and partially on homestead-based agriculture. For this reason, it has historically been poorer, more unequal, and more unstable than the rest of the United States. Second, all studies that have reached the conclusions above are purely correlational. There have been numerous events that threatened the stability of the Northern US, Canada, and Australia, and several examples of stable post-colonial countries that are commodity dependent, namely the UAE, Oman, Tanzania, and Gabon.

Sources:

Hoffman, Andre. The Economic Development of Latin America in the Twentieth Century.

Pantin, Dennis. The Plantation Economy Model and the Caribbean.

Rodriguez, Francisco. The Political Economy of Latin American Economic Growth.

Robinson and Conning. Enclaves and Development: An Empirical Assessment.

Fuentes, Stefania. Origins of Institutions and Inequality in Latin America.

Ochonu, Moses. Colonial Meltdown, 1st Ed.

Ochonu, Moses. African Colonial Economies: Land, Labor, and Livelihoods.