Because the alt-right has increasingly made it acceptable to be a blatant antisemite and continue to spout misinformation even after factual correction.
If you've been wondering why I'm especially pissed at aggressive internet atheists, let's talk about some history. 1/x
That in itself isn't a problem. So am I. So are a lot of people in the US who aren't Christian. Christianity is probably at its most toxic in the US right now, and evangelicalism is awful for living things.
Underlying this is Judaism's status as America's most familiar religion after Christianity...
But there's another component here, which is angry white men/4chan/alt-right/etc.
Women, "SJWs", liberals, minorities... if you're an Internet Atheist, "religious people."
And the vast majority of conspiracy theories end up with The Jews Are Responsible. Sometimes we're also lizard people, or aliens, or whatever, but we almost always end up playing the villain in these stories.
Internet Atheists are still PRIMARILY angry at Christians, so what's the problem?
Well, the problem, dear reader, is historically whether it's Christians angry at us or people angry at Christians, we still get killed.
Hitler started out by selling himself to the public as a Christian, but a BETTER, more enlightened Christian than following traditional Christianity made most people.
Jesus was an Aryan dude who was fighting against the institutional power of corrupt, materialistic, selfish Pharisees.
But that connection is right there, waiting to be made.
So publicly, he's promoting "Positive Christianity," a Christianity washed clean of its Semitic roots, in which Jesus came to save the world from the Jews.
What did Hitler actually believe, though?
As Kershaw points out in his biography of Hitler, he privately denigrated Christianity to those close to him.
1) it was a rival power that could undermine loyalty to them
2) it had Jewish roots
Angry white man raised in a Christian environment that posits Judaism as the problem Jesus came to solve decides the Jews have conquered the world through Christianity.
And any movement made up of predominantly angry white men pissed at Christianity whose understanding of who Jews are is shaped by a Christian background are likely to end up with a view of Jews as puppetmasters responsible for it.
It's not, we're already vulnerable to violence, and it very quickly becomes "The Jews are RESPONSIBLE for Christianity."