There are a few ideas. Anti-Semitism has been around for a long time.

One is that Judaism was one of the few religions that allowed money lending with interest. This was considered usury and prohibited in some other religions, namely Christianity and Islam. The idea being Jews would be money lenders, and would suffer the wrath of anyone who couldn't or didn't want to pay them back. In fact Jews sometimes played an important role in the economy of European powers, often they were prohibited from other trades. Leaders could tax/extort them or borrow money from them, bypassing rules on taxation or religious laws on lending. This could lead to persecution and expulsion, often without any real concern for their safety.

https://en.wikipedia.org/wiki/Edict_of_Expulsion

Ultimately they were a people without a nation or homeland. Humans aren't always the most welcoming of creatures, or the most fair. It's historically been easy to use Jews and other minorities as a scapegoat. We still see it today.

Keep in mind it's not really like today where we have minorities but they're at least ostensibly citizens with rights. Jews were often forced to live in ghettos, Jewish quarters or areas, or have identifying marks. There was no pretence, they were the other. It's not easy to break that sort of ingrained social structure.

I'm sure any Christian you asked might have had their reasons, but I think the fundamental reason, as depressing as it is, is simple: The Jews were often treated so poorly because they could be treated poorly, like any minority group. It was just their misfortune to always be a minority group.