Democrats are even more scared of people learning the truth about Israel and Palestine than Republicans are.
It's really a very simple matter - Israel's treatment of the Palestinians is indefensible. That's okay for Republicans, since they aren't going to bother defending them - they and their base revel in bigotry and hatred and violence, so Israeli bigotry and hatred and violence is fine by them. But Democrats have to at least pretend to be opposed to bigotry and hatred and violence, so they can't defend Israel from legitimate criticism. So their only hope is to protect them from it by hiding the truth.