1. We have a large Pro-Israel lobby, and they push for support. Not supporting Israel comes off as Anti-Semitic, so politicians are happy to oblige. The large Jewish population in South Florida also gives Jews a sizable hand in elections, which results in politicians not wanting to risk offending them. Also, as Jews are generally among the wealthier people around, they have more ability to "get what they want" politically.
2. Israel is a Democratic country that's very friendly to us in an area where we don't have a ton of allies. As such, it becomes very valuable, from a military and political perspective because of that. (See South Korea and Taiwan for other examples.