He needed someone to blame.
Antisemitism had existed for centuries in Europe, which meant that Jews were generally a people folks didn't like very much and were less likely to trust. Did you know that one of the reasons Jews became so prolific in had to do with Christian beliefs on handling money. Handling money was seen as a sin and so most Christians would never become money lenders, or money counters either, during the middle ages. Few jobs were open to Jews at that time, one of them being money lending. This meant that over time Jews gained some power thanks to money. Take the influential Rothschild family of bankers, they were themselves Jews who became a powerful family because they were bankers.
Now here you've got antisemitism and folks having to go to banks and other institutions largely run by banks. Then add to this WWI ending with the blame for the war being placed on Germany and thus Germany having to pay for the war. This meant a recession in Germany, which only became worse when the Great Depression hit. The Depression had an effect the world over, and in Germany part of that effect was to stop the flow of money from abroad that was to go to rebuilding the country. Unemployment went through the roof and folks were ripe for someone to lead them out of the dark days.
Into this enviroment steps Hitler. He'd been active during the '20s but the time was right for an entire nation to turn to him. And he knew just what to say. The people needed someone to blame for their troubles. So what does he do?
"You're not responsible for what happening to you now. Someone else is to blame. Why did Germany loose the Great War, it wasn't for lack of the fatherland's brave boys giving their all to win the war. No, they were sold out. And who sold them out? The Jews. Oh but they didn't stop at costing the fatherland the war, they deliberately mismanaged your money. It's the Jews fault your children are starving at home because you don't have the money to buy food. Not yours. No, never your fault."
The problem with this, of course, is that many of the brave boys that fought the were were also Jews. Jews fought just as hard as any other Germans in the war and were just as likely to get decorated for bravery. And post war they were just as likely as other Germans if they were not wealthy (and the wealthy of Germany were not only Jews) to suffer. But under Hitler they became scapegoats for the suffering of their Germans. Any Jewish soldier in WWI who was awarded a medal could have it stripped away from him as Jews couldn't possibly have been loyal according to Hitler.