why was is Islam, the fastest growing religion, missing from JW doctrine? - because some Muslims react extremely violently when their religion is disrespected. A glossy little brochure from the WTS arguing against Muhammad's teachings would certainly fall into this category.
Although having said this, I'm not entirely convinced of it. I had a conversation with a young JW when I was waking up. And this brother was defending Islam - he said, and I quote, 'they believe the same as us'. I tried to reason with him, saying although they reject some doctrines JWs do (e.g. Trinity, idol worship), they have beliefs of their own (e.g. Muhammad was the last prophet of God).
He couldn't see the point I was trying to make - it was like banging my head against a brick wall, lol.