Formal:
* Waarom nie? (This is the most direct and literal translation)
Informal:
* Hoekom nie? (This is a more casual way to say "why not?")
* Wat nie? (This is a shortened version of "Hoekom nie?")
With emphasis:
* Waarom nie dan? (This adds emphasis and can be translated as "Why not then?")
The best option to use will depend on the specific situation and the tone you want to convey.