Many, many reasons. Of course it has something to do with the fact that there are many Jews in the United States, but in my opinion this can't fully explain the relationship. I think it has more to do with the history of the United States, our values, and of course strategic reasons. Also you have to remember that Evangelical Christians are extremely pro-Israel and it is estimated that there are many tens of millions of them in the US.
It is very simple to say that "Rich Jews are the reason the US is pro-Israeli", but at the end of the day, despite the myths of Jewish power, it is extremely difficult for <3% of the US population to build and sustain the special relationship we have with Israel.
In the early years, it was very "in" to be pro-Israeli. Major stars like Frank Sinatra travelled to Israel and fell in love with the country. Israel became the country that everyone loved to love in the early years. It's estimated that 25% of the intelligence the United States received on the USSR
in the 1950s came from Israel. Israel delivered major intelligence scores for the United States by stealing Russian-built MIGs, and even obtaining a copy of a secret Soviet speach by Khrushchev that was extremely important to the West.
In the 1970s and especially the 1980s, as Israel's celebrity statues died down, Israel was a major ally in the Middle East. They were a democracy, extremely pro-American, had an amazing military and unlike say Iran, we didn't have to worry about arming them and then having a revolution. Israel became a major strategic ally in the 1980s...Once again as the United States faces major conflicts in the Middle East, Israel has again become a strategic asset of the United States.
Over the years, the low points were with Eisenhower and Bush Senior, with the best years (of the US-Israel relationship) being under Reagan, Clinton, and George W Bush.
Over the many years of relations, many ties have been built. For example, many American first responders now train in Israel. These people come back to their communities in small town America and by just talking about Israel, a bond is built. The more bonds that are built, the stronger the relationship is. Thousands of people, from aircraft technicians to military commandos, interact with Israel and build more bonds.
Perhaps these two webpages explain it better than I do:
"Millions long for immortality who do not know what to do with themselves on a rainy Sunday afternoon"