This is why:
Don't listen to those who impute their own morality to others - actually read and listen to the Islamicists themselves - they are out for Conquest. Individual Muslims may live peacefully in our country, and other Western countries. But, the radicals do not, will not EVER. Any concessions they make are intended to be a temporary measure, before they return to assault our civilization again and again.
But what Houari Boumedienne predicted at the U.N. in 1974 and what Muammar Qaddafy predicted in 1996, and what many other Muslims (you can find them online, at YouTube) talk and write openly about, that is, the demographic conquest of Europe -- which, as Qaddafy said, "does not belong to Europeans" but, by rights, to those who are the true masters of the world, the "best of peoples," the Muslims - is aided both practically and symbolically by the construction of mosques. And Muslims have a penchant for mega-mosques. Indeed, they have tried to build several recently in London, and only through great efforts by protesters have those projects been halted.
Why, you may ask? Why the huge mosque complexes? The reason is that Islam is a collectivist faith, and the spectacle of thousands of people prostrating themselves in unison is understood to be a kind of impressive sight. It is impressive, that is, to the Believers themselves who enjoy the crowd psychology of it all, the enrolling of themselves in a Nuremberg-rally mass, in which they lose whatever hint of individuality they might have had. Western man prays differently. He prays individually even if in a church with others, not as a mass that, in trained unison, bows to a Creator whose ways and decisions are never to be subject to moral debate, and whose rules, that is, the Total Islamic Regulation of Life, must never be questioned either. For to question one part of Islam is to potentially call into question the whole brittle mental and emotional framework of Islam, which cannot stand and will not tolerate free and skeptical inquiry.
The mosques of Rome and Grenada are not centers of "tolerance." There is no mosque on record either in the vast Muslim world, or among the thousands upon thousands of mosques that have been built all over the West, of a single one of these mosques being a force to encourage "tolerance" among its worshippers, or to "serve as a center of tolerance." Mosques have been used, as we all know, as places from which fire has been rained down on Americans and other Infidels in Iraq. And in the West, mosques have been raided in many different countries and by many different police forces. They have been found to contain false ceilings, where explosives, guns, false papers including false passports, and other suspect or dangerous items, have been found. Just ask any of the anti-terrorism heads or magistrates - ask, for example, Jean-Louis Bruguiere of France, now a consultant, thank god, in Washington (where one hopes his sober and no-nonsense views, based on years of successful activity, will find a receptive audience).