AIPAC money.

They donate to both parties primaries so by the time a general election comes around, no matter who wins they will support Israel.

They have zero qualms ditching a politician and supporting a primary challenger if they ever say a bad thing about Israel.

Currently it’s also because the US president is a self proclaimed Zionist who has went on the record for decades saying nothing will ever stop him from supporting Israel.

Israel is a potential military staging area for the US in The Middle East.

Once you have a powerful enough lobby, you can do anything to anyone, without repercussion. Israel is testament to this