To go back to your question in the title, "Why did it happen?"
Well it was written and it happened, and the agencies bringing it about are irrelevant.
From the centuries of Jewish persecution, the amazing coherence of a nation without a land, the Russian pogroms, Theodore Herzle, WW1, the Balfour Declaration, the rise of Hitler, WW2, the Holocaust, the British Mandate, the U.N. vote, to name just a few, are all the circumstances brought about by Gods will.
That is why it happened.