Often that's because anyone who asks the question (regardless of the legitimate points they might raise) is immediately accused of being anti-semetic.
My short answer would be:
1) Very influential/powerful U.S.-based pro-Israeli lobby groups (i.e. - JINSA), that are often highly-connected within the U.S. government.
2) An "allied" nation which serves as (essentially) an extension of the American military. The U.S. taxpayer provides several billion dollars in aid (earmarked for miltary purposes) which Israel uses to purchase weapons and defense systems through American manufacturers and contractors (I tried to talk about this in detail with LANMaster a couple of months ago, but I can't remember which thread it was).
Israel also tends to reverse-engineer much of this American military technology, which they then turn around and sell (helping them to develop their own defense industry). I believe that China and India are two of their biggest clients (although this landed them in some hot water with the U.S. a couple of years ago).
3) Tied in with #2, of course, the relationship is very much one of necessity. Israel's strategic proximity to some of the largest oil reserves in the world make it that much more valuable to the U.S.
In terms of policy, Israel is fairly malleable. Israel is technically a sovereign nation, but only a fool would think that the U.S. doesn't possess a large degree of control over the country.
I think it really comes down to business/money and the security of overseas interests (which is also about business/money).
For all of our talk about human-rights and democracy, the truth is that the almighty dollar was/is/will always be our primary concern.
Where money is concerned, innocent men, women and children stop becoming people, and are instead relegated to the status of "collateral damage".
Sad but true.