A whole host of reasons, some old and some new. In no particular order:
During the Cold War, Israel was a U.S. client while its neighbors - Syria and Egypt in particular - were Soviet client states. This became particularly important after the fall of the Shah, as Iran left the U.S. sphere of influence.
The United States' Jewish population is much larger and far more influential than its Muslim population. This was true to an even greater extent in the past.
A wide-spread perception that the Israelis are the 'good guys' and 'like us' because they speak English/are of European ancestry/have a democratic government, etc., while their neighbors do not.
Decades of random, world-spanning terrorism by the PLO and similar Palestinian organizations.
Worry as to what a nuclear-armed nation in dire straits might do.
Jewish supporters of Israel in the United States are a significant political block.
The religious right apparently views support of Israel as an aspect of their faith.
Because there is a very strong and very vocal pro-Israel lobby entrenched in Washington D.C.
Because the United States guaranteed Israel a certain level of support under the Camp David accords.
Because the United States and Israel have been allies for long enough that support for Israel has become the status quo.
Yeah, that's pretty much it - a lot of complicated reasons.
I was going to add military intelligence as a big one, but that's implied in this post.
