For two reasons - First, I don't believe the NVX XQW is on most people's radar as the stellar performer that it is and second, I don't believe it should actually be classified as a shallow mount subwoofer. It's just a normal build that's isn't as deep as some other offerings.
Either way, the XQW definitely does not suck donkey bawlz.
Edit: You meant the ID not the NVX, sorry about that.