1. The economy. Economic growth (which Podhoretz associates with conservative, business-friendly economic policies) is good in itself and also leads to a culture of affluence in which economically-successful minorities such as Jews are respected rather than resented.
2. Social issues. Socially conservative attitudes are more consistent with the Old Testament values of Judaism.
3. Israel. In recent decades, Podhoretz argues, the Republican Party and its allies in the conservative Christian evangelical movement have been more supportive of Israel than have the Democratic Party and its allies on the left.
I'll take these in turn. As for economic growth, Andrew notes that basically everyone is in favor of economic growth. There's no particular reason Jews should like this more than any other group. Also, there's no reason anyone should prefer the Republican record on economic growth to the Democratic one. Both parties push for growth when in office, and Democrats actually have a better record of producing it for a broader segment of the population.
I'm blanking on the exact source, but I once read an interesting discussion of Jewish voters by a former member of the California Assembly.* As he argued, unlike most minorities, Jews don't become more conservative as they start to make more money -- Jews aren't so quick to abandon old ideas once they become unfashionable, which probably accounts for their still being Jewish.
Second, people's social stances aren't necessarily determined by the stances of their religion's founding documents. And to a large extent the values articulated in the Old Testament don't really map onto today's world very well. If you believe a person should be put to death for wearing two kinds of fabric or for insulting his parents, or if you think a man should be banished for having sex with a menstruating woman, which party do you belong in today? I don't think you'd be comfortable in either one. Besides, another ancient tradition taught by Judaism is social justice, one that probably maps onto modern liberalism better than onto conservatism.
And as for Israel, Republicans have been more supportive than Democrats of Israel in recent years only to the extent that they keep saying they're more supportive than Democrats of Israel. Leaders of both parties have gone out of their way to provide assistance to Israel. Also, some Jews may be rightly leery of Christian Evangelical support of Israel, since such support is designed to help bring about the End of Days when Jews and other non-Christians are burnt to a crisp.
*James Mills, A Disorderly House: The Brown-Unruh Years in Sacramento, 1987.