If you're a good atheist, you'll know why.
I could take a couple of newspapers and some DVDs with me in a time machine back to 1942 and within 8 hours the entire British army would have defected to the German side. I'm no fan of WW2 Germany, I'm telling the unvarnished truth. All I would have to do is to show them what would happen to Britain after the war was over. They would tear off their uniforms, throw down their guns and run to the German side asking them how they could help. That's the honest truth.
The fantasy of multikult is that you can put millions of people with no common culture, no common interests and no common context into the same land mass and they will be good neighbors "just because." That's a religious faith that is more irrational than the ancient barbaric practices of the Druids. Such affairs always end in rivers of blood and babies, the other white meat.
If man learned from history, the world would look radically different from the way it looks now. Man does not learn from history because man is not a learning animal. 95% of the population can no more learn or reason than can a popinjay. They are just bright enough to put on manpants when they get up in the morning but it ends there.
This would be an unrivaled slaughter in the making in the best of times. Throw in a couple salts like natural climate change and synchronize it with the decline of the West, you've got the Apocalypse Trifecta.
There have been multiracial societies that have prospered for a short while, as long as the people in them were strongly bound by rigid mutual principles and shared religious faiths. We know about them in history. There are no records of prosperous multicultural regimes in the pages of history because none of them ever lasts much more than a generation before they are destroyed either from without or within. The stakes are higher in the modern world. The barbarians will be packing nuclear weapons this time when they breach the city gates that nobody can be found to defend.