I've wondered about this as well and I suspect it's either or some of each of the following three:
Religion: People who are Jewish - this one is pretty straight forward. People who are anti-Muslim, not necessarily a full blown bigot, but just have a less favorable view of the religion. People who are Evangelical Christian - I'm less familiar with this, but Israel plays a big part in their prophecies, so Israel must be supported and protected now, so that the prophecies can come true, or something like that.
Domestic politics: If the perception is that Democrats are for Hamas, for Palestinians, and against Israel, then the right/GOP/conservatives will be on the opposite.
Age: Older people may remember growing up when Israel was the upstart underdog that was vulnerable to their more powerful neighbors. Israel was a compelling recipient of our patronage. Of course things have changed, but people's experiences and views may not have.