I've seen this question come up a few times before, as at least on the surface it seems rather peculiar-- yet if we start thinking about what those former colonies have in common, the answer becomes relatively clear. Fyi, I am leaving Singapore on the side as its position as a trading center/ regional financial center is a different story altogether and not one I know as well.

So what do the remaining USA, Canada, Australia, and New Zealand have in common? First off, they are all settler colonies. That is colonies that saw large influxes of white European males/families. These settler communities saw themselves as European (later 'white') and therefor expected to achieve similar standards of living as in Europe; they expected parallel political infrastructure, they were by and large not governed as an 'associated' or 'savage' people, but were governed as actual citizens of the Empire of which they were a part. By the late nineteenth century, and much earlier in the Americas, European descended populations dominated demographically Australia and New Zealand; this is very much unlike the British colonies in South Asia or Africa, wherein the white settler population was small to non-existent, especially compared to indigenous populations.

Each of those colonies also saw large scale land alienation from Native peoples and dramatic population declines caused by disease, warfare, and systematic state sponsored violence/confinement. This left a lot of land to be sold and acquired for ranching, farming, settlements, and available for extractive industries like logging and mining. That last one, mining, has played an exceptional role in the wealth generated by Canada, the US, and Australia in particular with each having had multiple resource rushes over the past two centuries for gold, copper, nickel, coal, oil, etc. These settler colonies were able to benefit from the development of local industry which they could own and participate in-- something very unlike many of the non settler colonies in which locals were only employed in 'subaltern' or basic positions, leaving all the highly trained or specialist roles in the hands of temporary imported workers. Michael Adas in "Machines As the Measure of Man" has a great analysis of the ways in which the ability to use technology was perceived by Europeans as a mark of progress, yet they very often systematically denied subject populations access to technologies; which Daniel Headrick in "Tentacles of Progress" argues made technology less 'sticky' in many of the colonies, especially during decolonization, as a nation found itself short of experts/ trained professionals. Suffice to say, this was never the case in any of the aforementioned settler colonies.

So there you have the long of the short of it. They are wealthy first world nations because of white settlement and land appropriation. None of those places were 'decolonized' or used primarily as simply a place to extract raw materials, but were actively invested in and built up-- though within those colonies, your milage may vary.

Edited to add: Here is an answer I gave to a similar question