I think because the press are like a pit bull with a bone in its mouth, they just will not let go and shake out every bit of 'proof' they can find.
Also, the fact is, moving to the left as the party has with Corbyn will attract more 'fringe' people, including those who think the Rothschilds are running the world, and so on.
I don't believe Corbyn is actively anti-Semitic, but I do think on some level he believes the Jews are doing all right and don't really need help and support.
As a Jew myself, I do not feel threatened in any way by Corbyn - the Holocaust didn't happen because the Nazis failed to manage anti Semitic elements in their party! The anti semitism was part of their raison d'etre.
I am also a Jew who opposes Israel's treatment of the Palestinians, but has never been comfortable going on protests, as there are too many parties that make me feel uncomfortable, something which has been going on long before Corbyn (people with banners about destroying Israel, people comparing Israel's behaviour to Nazis - yes there is some truth in that, but it is needlessly hurtful and alienating to Jews and will change nothing- and so on). So yes, the left has its own problems with Jews but I'd like all sides looked at, thanks very much.