Because it represents a mystical belief system that rejects many aspects of modern western society. Because of this it creates very visible divisions between people and on a larger scale society. Because it's a belief system it can't be argued with, reasoned with or adapted - it remains a growing, intransigent corrosive impact upon a society that is known and accepted.

In addition to this the fundamentals of this belief system are abhorrent. It is only by wilfully ignoring sections of it - in much the same way as the new and old testaments in Christianity work - that these fundamentals become less important and a greater meaning of peace, unity and love is derived. While its laudable to want to view a belief system through this prism that is loosely called 'moderate' what essentially happens is people become a support structure for a set of ideas that's incompatible with a modern society. The idea of Jihad. The head and face scarf. The separation of women. The intolerance of homosexuality.

To many the pacifist and 'moderate' support of Islam is much the same as the way in which you would view supporters in the past of political ideals that turned out to be a cause of death and suffering. Would every supporter of National Socialism be responsible for the murder of millions in the gas chamber? Are we saying that every member of the Nazi party is responsible for this? Of course not. But to be a Nazi party member, to wear their symbol, to even have similarly aligned political ideals is abhorrent. We as a society reject even far removed similar political ideas knowing to what purpose they can be bent. Society knows that where these ideas begin, only pain and suffering end. It is through this prism that Islam is rightly seen. As a secular person if you reject the very idea of God then you would surely reject even the passive support of a belief system that is unarguably the driving force of pain and suffering across much of the world.

So to sum it up:

1) it divides society in a very visible way with seemingly society moving to accommodate the faith more than faith is accommodating society 2) the basics of Islam are incompatible with the changes in society of the last thirty years 3) if you reject the idea of God - and many do - what Islam represents is a set of ideals in whose name terrible pain and suffering is perpetrated. What compounds this is how widespread it is and how seemingly indestructible - more Muslims die to other Muslims than any other race or religion. By believing and practicing you are perpetuating the cycle.

I of course expect people to disagree but I've tried to set out what I believe the reasons are - reasons that for the large part cannot be discussed in the open for fears of racism and other misguided criticism. Open debate about ideas is important and Islam is an idea fundamentally.