How did Dutch in southern Africa change into a new language while American English and British English are completely mutually intelligible? What differences in English and Dutch colonialism or the different environments maybe caused this?
hi! there's room for more info, but you can get started on this now-fragmented post
The Moderator's link is pretty extensive on the details of Afrikaans, and what influences caused it to more drastically diverge from standard Dutch.
I want to add that you should be aware that there is no clear divide between what is a dialect and what is a different language. Mutual intelligibility is a lot vaguer in practice than it sounds in theory.
For example, there are many dialects in the Netherlands whose sounds are more different from standard Dutch than Afrikaans' are. But at the same time those dialects' grammar and vocabulary are near identical to standard Dutch while Afrikaans' differs considerably.
In reality the main decider between dialect and language is politics. Within countries variations will be called dialects, between countries they'll be called languages (unless of course, as is the case with British and American, the differences are so minute calling them different languages is untenable).
Though if you care for my opinion as a native Dutch speaker with some elementary knowledge of linguistics, while I can understand large amounts of Afrikaans without ever having studied it, I find the differences big and fundamental enough to justify calling it a very closely related language rather than a dialect.