(1)They fear forced religious assimilation or proselytization by the government, which is perceived to be a problem for Jews gong back for centuries (Spanish inquisition, diaspora, etc). Thus, perhaps irrationally, they fear any attempt by religious conservatives to "impose" what they perceive to be religion-based social views through the political system (such as anti-abortion views). I don't know for sure, but would suspect that Jews actually have fewer abortions per capita than some other groups. To liberal Jews, it is the principle--no religion-based governmental mandates--that counts more than the underlying substance.
(2)Many American Jews of Eastern European descent are only a few generations removed from family members that were active socialists or communists (in the period 1900-1940), and were raised from birth on ideals of "social justice" drawn from those movements. In some ways, the lofty ideals of socialism became a substitute religion for those who did not formally practice their Jewish faith. Many of these Jews overlook the terrible practical results of socialism and communism for Jews and the general populace the Soviet Union and elsewhere because they did not experience that in the US, so reality has not tempered their ideals. <3> Jews have been active members or supporters of the civil rights movement since its inception, believing that when racial discrimination is eliminated by the government, society also will not tolerate religious discrimination. They may be very wrong about this connection, unfortunately.
Until Jews become willing to reassess these three deeply-seated beliefs, I believe that conservatives will have tough going in the American Jewish community.