Tl;Dr - Massive unopposed lobbying campaigns for the last couple decades and the Cold War.

Honestly? Domestic politics. Israel has had an influential lobbying arm in the US since World War 2. Combine that with post war guilt about turning Jewish refugees away during the Holocaust and the cold war position of Arabs and you have a slam dunk case for the US backing Israel until the mid 90's. Which is what you see in history.

However Bill Clinton correctly identified the political time bomb inherent to Israel after peace talks fell through and began pressuring them to end new settlements. The lobbying groups helped get George Bush elected because of that. (To be clear these are Americans, some Jewish, some not, that believe in Israel for varying reasons. Not Isreali agents or anything like that.) George Bush also did not want that political hot potato and kept Clinton's pressure going. Especially as he tried to rally support for a second war.

At this point around 2003 alarm bells started going off in the pro Isreali lobbying groups and in the Israeli government. They sensed that now that the cold war was over they would not receive special treatment anymore. So they began laying the ground work for major lobbying efforts at the city, county, state, and federal levels. They also began a pressure campaign on media to have issues presented on a light partial to Israelis. In 2004 conservative Israelis and their allies had a taste of their worst nightmare. The Israeli prime minister ordered settlements in Gaza to be abandoned. This was because of international pressure.

Netanyahu and the pro Israel lobbying groups proceeded to work at making America a shield for Israel over the next 15 years. Resulting in many states and localities passing laws to prohibit the BDS movement and define any criticism of Israel as Anti-Semitism. This is known as the IHR definition which the US House just adopted.

While there is a significant portion of the US population that supports Israel as a result of decades of cold war propaganda, they aren't why the government is so vehement about it. Israel and its friends have lobbied our government directly for nearly 20 years without much resistance. Pro Palestinian groups, and positions have been seen as largely terrorism aligned in a country that's been at war with groups labeled as terrorists for 20 years. So any resistance that was encountered was easily dismissed.

That's why our actual infrastructure of government favors Israel in places. Now why are we so actively supporting them instead of just letting the government do its thing? That comes down to one man and no one knows his reasons but him.

Other suggestions -

Christian End of The World Theology- This explains some grass roots support but not the majority of grass roots or the government's support. The US government has historically been hostile to this theology.

Oil - This doesn't track. We're in a big push to be independent of foreign oil and trying to transition to green energy. It could have been a reason 20 years ago, but not now.

Military Asset - This was true until the cold war ended. Now the US has bases all over the middle east and Turkey, a NATO ally, is emerging as a competent weapons manufacturer with far less support than we give Israel.