Waarom nie?
It is important to note that Afrikaans is a flexible language, and depending on the context, there are other ways to express "why not." For example:
* Wat is die rede om nie? (What is the reason to not?)
* Is daar 'n rede om nie? (Is there a reason to not?)
* Hoekom nie? (Why not?) - this is a more colloquial way of saying it.
The choice of expression depends on the tone and formality of the conversation.