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Transportation was a large issue: I unfortunately don't have a citation, but my notes from a class I took a couple years ago are as follows:
There were 2 ages of Imperialism: Pre-industrial and industrial, the 2nd age beginning in the late 19th century.
Important differences in overseas expansion efforts between the pre-industrial and the industrial eras
One reason could be that the late 19th century just isn't that long before WWI, and the beginning of the end of direct Imperialism. That isn't much time to set up colonies.On the other hand Britain managed to heavily colonize North America and Australia before this time, and likewise, Spain did South America.
If you look at Africa as well, there were instances of direct colonization, most notably the Boers of Orange Free State and Transvaal whom were of Dutch descent, and the colonization of South Africa.
Upvote:5
Just to expand on the disease angle, at its height, Zambia's population was 3% European. In contrast, Nigeria's European population was only 0.04% with West Africa in general described as "white man's grave" due to the large number of deaths caused by yellow fever and malaria. Even within Africa you can see the dramatic difference.
Upvote:10
Adam Smith in the Wealth of Nations has a short section on this question. He explains that while Africa was "inhabited by barbarous nations" just like the Americas, the former were primarily shepherds while the latter, with the exception of Mexico and Peru, were hunters. Because a society dependent on shepherds can produce more food and sustain a higher population than that dependent on hunters, they were therefore more difficult to displace. The Dutch settlements in South Africa were possible because the area was "inhabited by a race of people almost as barbarous and quite as incapable of defending themselves as the natives of America." There is no mention of Australia because of when the book was published but the same logic could be applied there.
Upvote:12
Germs.
The natives of Australia and the Americas were wiped out (some places seeing ~95% fatality rate) from smallpox and other diseases that the Europeans carried to them. In some places the germs went faster than proper colonisation. For example, the natives of the Mississippi seem to have been wiped out after a handful of Europeans made contact with them.[1]
Once the land is empty, it's much easier to settle.
Compared to the Americas, the Africans had their own diseases (malaria) that would have killed Europeans and had intermittant contact with Europeans for centuries (and so would have gotten contact with the diseases).
1. Mound builder (people), Reports of early European explorers. Wikipedia.