"Kwa nini basi?"
I don't really agree with the "Why is that all?" translation, which is very ambiguous in English. I see "Kwa nini basi?" as something that would be said after a long series of questions about the reason something happened or is happening, and each question receiving a "No" answer. E.g.: Kwa nini unalia? Unaumwa? HAPANA! Una kiu au njaa? HAPANA! Umepotea? HAPANA! Umepigana na mtu? HAPANA!! Kuna mtu amekutukana matusi mabaya? HAPANA!!! Kwa nini basi unalia???
In short, I think its meaning is "So why then?" or "Well then why?"