Plain and simple question, just looking for the answer.
According to the Crisis of German Ideology by Mosse, the völkisch ideology, which the Nazis had a particularly popular flavor of, was inherently racist; each race had a natural destiny and a particular connection with their land; I.e. good yeoman stock. (I may mildly missummarize here, I am compressing 300 pages. Quite open to corrections)
Since the Jews were rootless, they were not Proper Folk; since they were Other in Germany and Germany had not achieved what was seen as it's Proper Glory - Something - Somebody must have caused it. The Jews! They were eventually seen as the Great Enemy (of course, this is within the völkisch ideology. Specifics varied! )
And, the lack of them being Proper Folk allowed the mistreatment.
Edit: here's a 60s book review critiquing Mosse's book - https://home.isi.org/metaphysics-nazismbrgeorge-l-mosse-ithe-crisis-german-ideology-intellectual-origins-third-reichi . Just to give you some flavor of possible issues with my explanation!
Well throughout history Jews have always been subject to persecution. During the Black Death that raged throughout Europe a good portion of people blamed the Jews. As a matter of fact Jewish ghettos were created around this time to contain them and the plague they carried. So racism against the Jews was nothing new.
As for the Nazi's well their hatred grew out of a long history of racism. Also they had their own ideology that promoted Jewish hatred. The Nazi's believed in the right that they were biologically chosen to become superior, Hitler helped promote this idea as well. They believed in cleansing the land for their own "Living Space". The Jews were unclean and they were polluting their living space. They were not as biologically superior to them. They didn't see them as human, they saw them as sub-human. Nazi Propaganda promoted this belief as well.
Along with that the Nazi Party was able to pass this belief onto the average German by blaming the Jews for the loss in WWI. The Jewish people, who in fact were wealthy and had great power over the factories and banks, were the ones that cause the German people to loose. While the average "pure blood" German suffered during post-WWI the Jews were well off. As well as that the Jews had their own tight nit communities. They remained, for the most part, wealthy after WWI and didn't quite suffer as the other half of the country did. So it was easier than usual to persuade people into hating the Jews simply because, well the Jews didn't really "fit" into German culture. They shared different customs, a different history, and they had a different living condition.
As for Hitler, well historical brainwashing and sheer evil led him to believe what he believed. But background content and current conditions made it easier for him to spread his hatred.
While the question might seem plain and simple, the answer is complex and complicated and has been addressed in various form in this sub already. Please refer to our FAQ as well as to some of these questions I answered in the past: