I had to learn this in school for the whole year. And to put it simply, the same as reasons as now. He thought the Jews controlled the media, the banks, everything. But at the time, Germans were poor while others thrive, specially Jews. So his reasoning was, Germans are poor because Jews are basically robbing and exploiting Germans, so he gathered the Germans to work together and be better as a race and punish them who made the Germans struggle.
And we all know how that ended. It is a stupid reasoning off course.