The answer is simple: To many leftist Jews, a non-leftist Jew is by definition a traitor. It's as simplistic as that. To these Jews, "left" and "Jewish" are synonyms. Just as to many on the left, "left" and "black" are synonyms.
The use of this smear is the tactic of those who cannot argue. Blacks and whites who smear black conservatives such as Thomas Sowell, John McWhorter, Larry Elder, Walter Williams, and Ward Connerly as "Uncle Toms" do so because they cannot, or wish not, to engage them in intellectual argument. It's far easier to libel them than to debate them.
The same libel and intellectual shallowness characterizes the Rosenberg piece.
His major charge is that Jews who support conservative Christians are "Jews taking positions hostile to the Jewish people in order to stay faithful to some political agenda."
And what are these positions that are "hostile to the Jewish people"? Rosenberg enumerates them: "the entire Christian Right agenda: opposing abortion, poverty programs, progressive taxation, laws that protect gays, affirmative action, the environmental movement, and feminism.
Even if that portrayal of conservative positions were accurate, why are these positions "hostile to the Jewish people"? Because they are conservative positions! Again, to such minds, "left" and "Jews" are synonymous, and therefore "conservative" and "hostile to Jews" are synonymous.
If you think most abortions are immoral; that a lower tax rate is better for society, including the poor; that the problem of poverty in America will not be solved by the government spending trillions more; that marriage should not be redefined; that a race-blind society is a finer society and that race-based affirmative action hurts both the recipients of the lowered standards and the society at large; that we desperately need the oil from a small percentage of the Arctic National Wildlife Preserve because doing so will help us rely less on Saudi oil and won't hurt any caribou; and think that, in retrospect, the feminist movement (as distinct from the belief in man-woman equality, which every religious conservative I know holds) hurt more men and women than it helped -- you are anti-Jewish.
Such shallowness is based in part on ignorance. For example, Rosenberg writes that "He [Prager] conveniently ignores the fact that Christian Right support for Israel is largely based on a religious belief that Christ will only return after Jews are all in Israel accepting the divinity of Jesus Christ."
That is the lie about Christians that the left spreads to prevent Jews from knowing the truth about Christian support for Israel: that it is rooted overwhelmingly in the beliefs that God promised the return of the Jews to Israel, that Christians are grafted onto the tree of Israel, that God blesses those who bless the Jews, that Israel is a humane democracy and its enemies are bloodthirsty and backward regimes.
To Rosenberg, conservative Christians are a caricature.
Rosenberg's charges that Jews who support Christian conservatives do so from nefarious motives and that Christians who support Israel do so from nefarious motives is typical of the left. They judge motives, not deeds. And the reason is clear. They are so certain of their moral superiority, they can only deduce that all those who differ with them are bad people. That's how a Jew who has devoted his life to the Jewish people can be called an Uncle Jake.