There are a few reasons, including:
* Modesty (regarding explicit subjects, the Qur'an tends to be more modest and address men specifically as well as use euphemisms, even if both men and women are intended)
* Generality (zawj/azwaj can be understood in some cases to mean "spouses" in general and not specifically "wives" as it is often translated)
* In some cases, it is specifically addressing men and speaking about women (for instance, the verse about zihar when it talks about men divorcing their wives via zihar)
So it really depends on the verse. A good resource to look at for varying interpretations of verses is _The Study Quran_ edited by S. H. Nasr.
Hope that helps!