"And if not, why?"
Translation:Et sinon, pourquoi ?
The word sinon = si + non = if + not. It can also mean else or otherwise. Just as Ripcurlgirl said, you cannot have si non in a sentence, just as in English, you can't write I can not.
Actually, you can have si non when non is part of a construction, like non seulement = not only, but that is really rare.
sinon means "otherwise, if not, or else". (It can also mean "except, other than, but, save" depending on the surrounding context.)
However, in reverse, it appears that the English expression "if not" only translates to sinon. Here, sinon and "if not" are both conjunctives.
I don't have a concrete answer for this but in my opinion it's because Et sinon, pourquoi ? is always use as a follow up question. Exemple : Devrait-t-il faire ceci ? Et sinon, pourquoi? In english : Should he do this? If not, why? You have to answer the first question by a No which is why it's a Sinon and not a Pas.