Answer:
Many reasons
Step-by-step explanation:
Jewish People were targeted and hated long before Hitler was even born. Antisemitism was nothing new when he showed up and began his political campaign. Churches (specifically Christian churches) portrayed Jews as unwilling to accept word of God, as agents of the devil, and as murderers of Jesus. Jewish people were accused of murdering children for religious rituals, causing plague, and conspiring to dominate the world. Obviously, none of this is true but back then, especially before media was prominent, people listened to politicians and the church.
In the 19th century, Jewish people were classed as a race and even now that ancient hate caused by lies remained. It was believed that even if a Jewish person was converted to Christianity, they still were evil as they had "Jew Blood"
Wounded German pride was to thank for Hitler's quick following. Germany's loss was pitted on German Jews, despite being a small population of the country, Hitler claiming that Jewish people had stabbed them in the back. This, obviously, was not true but it got the ball rolling. Mass debt and poverty had the country on its knees after the war. People were suffering and needed someone to look up to... Hitler was that someone, providing a new hope for German people. He brought back their pride and strengthened the country once more. People trusted him and Jewish people suffered as a result.
Historians are unable to pinpoint his antisemitism to one specific event. It is actually unknown specifically why he had such a grudge against Jewish people.