As much as people hate to hear the "it's complicated" answer in regards to anything regarding Israeli affairs, well, it's pretty complicated... But to name a few reasons:
Religious fundamentalism/Zionism in the United States has been forcing politicians to take sides or else alienate their constituents - Christian fundamentalism is bizarrely in favor of a Jewish state because it fulfills their own doomsday prophecy, not for any logical reasons or out of a genuine desire to help the Jewish people.
Weapons manufacturers and arms dealers are lobbying to keep Israel as a partner considering they are almost always mired in some kind of conflict. It's a multi billion dollar industry and the people who make guns, bombs, artillery shells and ammunition are not keen of giving up their cash cow.
Consider of course the historical ties and diplomatic agreements, which if flippantly broken send a message of unreliability to other allied nations (I'm not sure I totally agree with this one, as the USA is basically a schizo nation flipping back and forth once every 4-8 years between isolationism and globalism depending on which party holds office).
Lastly, since the Cold War, the USA has kept Israel close to project their own influence in the middle east. Countries like Egypt or Saudi Arabia would have to think twice about provoking the United States considering who their neighbor is. Great strategic location bordering the Mediterranean Sea that makes it easy for USA to get soldiers on the continent if it ever comes to that.