Quinoa

I was looking through our dictionaries to see if they could throw any light on why it is spelt one way but pronounced completely differently. My first dictionary (Concise Oxford 1964) doesn't list it. The next (Oxford Compact 1996) doesn't list it but the 3rd Edition of the Oxford Dictionary (2010) does list it and says it is from the Spanish, so Qu must be pronounced k in Spanish etc. It doesn't say what the original South American spelling was, if they had one.

My thinking was, if it is a new English word, why isn't it spelt like it is pronounced.