One reason is the major change in how the Watchtower once pressured Jehovah's Witnesses to be competent in making a "fine witness" through "personal study". Witnesses were expected study on their own to make their public ministry more effective. In this case, Witnesses would have to study Islam to effectively engage on the topic with Muslims.
The Watchtower has changed its stance on this. Personal study has been reduced. Witnesses are actually discouraged getting into deep topics of faith. The fear of the Watchtower is that Witnesses will find out the truth about the Truth.
It is easier to witness to persons who profess to be of a Christian faith (common belief system) and simply ask them to visit the JW website or watch a video.
The Watchtower knows good and well that new members have predominately always been the children of current members. Out of fear of current members becoming educated and leaving it is safer to limit witnessing.
Side Note:
There are 3 Abrahamic religions - Judaism, Christianity, and Islam. All 3 are manmade religions. Christianity simply hitched its wagon onto Judaism, which is a manufactured story of a people who were desperate not to lose their national identity from being conquered by every world empire through history.