Because Israel is the most important partner of the United States in the middle-east. It's the only country in the region with a value system and culture similar to that of the United States.
The only other important geopolitical ally of the United States in the region is Saudi-Arabia, but that partnership is quite unstable due to the extreme ideological differences.
So losing the support of Israel would be very detrimental to the geopolitical interests of the United States in the region.
Also, many US companies have close ties with the Israelian economy, especially with the high-tech industry in and around Haifa and Tel Aviv. Severing these ties due to diplomatic differences would be a strong blow to the US economy.