Probably because making porn is haram, thus Muslims don't do that? Also, Muslims are much more defensive about their religious image. Like you can make Jesus Christ a slut and it'll probably be fine, but if you dare to draw a face for Muhammad, you will probably get death threats.
Honestly, I am more confused that you expected there to be a hijab in a porn game. It make women look bald, which is not really appealing to the playerbase, vast majority of which are not Muslims.