More than a dozen reasons, at science fiction site io9. An example:
Making History by Stephen Fry: A history student and a physics professor manage to send a permanent male contraceptive pill back in time where Hitler’s father will consume it, ensuring Hitler will never be born. But without Hitler, the Nazi party is ripe for the leadership of Rudolph Gloder, who shares Hitler’s genocidal agenda, but is far more efficient, stable, patient, and charismatic. Free from Hitler’s personality flaws, Gloder was able to take over all of Europe, so that, in the alternate present day, an extremely conservative US is in a cold war with the Nazis.