I don't know the specific rules in regards to Arabic, but someone sometime defined specific rules about how foreign languages, especially, with non-latinate alphabets are to be transliterated to English. These rules were set up so that when I write Iraq, you know what I'm talking about, and not everyone is transliterating it in their own way (which would otherwise be perfectly legitimate, since the sounds used in Arabic simply don't exist in English).

Why exactly whoever it was chose the `q', I don't precisely know. I am pretty sure that the sound used at the end of that word does not precisely match any sound we use in English. It lies somewhere between `k' and `g', which, if you think about it, is kinda where `q' would be if we ever pronounced it without the following `w' sound indicated by the `u'. Then again, not so much.

To make a long story short (too late), arbitrary rules.