I thought about posting this to /r/linguistics but the answers there tend to be a lot less historically focused and much more to do with the science of linguistics which often just confuses the heck out of me. I'm interested in the historical background to this.
Edit: to clarify- I know Afrikaans comes from Dutch not English, I want to understand why it is not still Dutch, while people in some former British colonies still speak English and not a new language descended from English.
hi! not discouraging anyone from contributing more information, particularly regarding English as spoken in South Africa, but meanwhile get started on some previous discussions about Afrikaans ~
edit: there have actually been a few chatty non-technical discussions on Afrikaans in /r/linguistics that could be worth a look
Afrikaans and the evolution of languages (x-posting from r/askhistorians) - includes a wiki link that compares a few phrases in Afrikaans to Dutch and English
This isn't a historical answer to your question, but Afrikaans is an offshoot of several Dutch dialects. So the reason Afrikaans doesn't resemble the English of New Zealand and Australia is that it isn't from English.