Mostly because one functional atomic weapon (I.E. not fission, fusion) could wipe Israel of the map. More than half of Israel is desert, so one bomb can irradiate the rest to such a degree as to make it certaintly a place you woulnd't want to live, and a smaller area unlivable for sustained periods of time. Now of course that samll area would mena less to big countries like the US, but that small area could be Tel Aviv, whose metrpolitian area is home to 1.2M people ou tof Israel's 7.8M.
Secondly because there are elements among the Iranian government who are not rational actors, and hence would not fear retaliation. If you allow Iran to have a nuke you are betting that the rational elements of the Iranian government trump those who would rather see the Jews destroyed at all costs. Iran is a Theocracy, and as we all know religion is irrational. With no fear of retaliation if those elements where able to have a nuke launched, the above situation would occur, effectively making the 50% of Israel that is live able undesirable, to say the least.
Also Iran represents a leader for Shia muslims in the middle east, who are by and large unfavorable to the US. The US prefers the dominance of Sunni muslims in terms of regional dominacne whenever possible, as the Sunni Muslim comunity in the Middle East is dominated by US allies, compartively to say the least, Egypt and Saudi Arabia. Iran has hated the US since the Islamic revoloution, so the Us is trying to limit it's dominacne in the region. Having a nuke gives you a lot more power in the international comunity, so in order to prevent an increase in Iranian power, the Us tries to stop them getting a nuke.
By the way this has very little to do with the Jewish vote 60% of Jews vote Deomocratic anyway so as long as Barack Obama doesn't say he would side with Iran, he will win the Jewish vote. Only a small minority of the US Jewish population are hard-core advocates for Israel, and as Flying-Dice mentioned they are a powerful and influential minority, given their size.
Edit: might doesn't make right but it does help protect your intrests in the Middle East.