Because all through history people find it much easier to pick groups of people as being the reason for all their problems, rather than blaming themselves. This might sound a little too simple, but it's the basic reason. In the case of Jews, it is due in large amounts to whose who controlled wealth, and people like Hitler encouraging anti Jewish sentiment for largely financial reasons, that the average person didn't know about. Hate of any group of people is entirely irrational, and it is something that is one of the most horrible things that continues to exist. The idea that you can blame someone for problems simply because they were born in a different setting to you would be laughable, if it weren't so real.