In part it's because Roberts himself, whether by design or principle, maneuvered the Court out of the line of fire with his split-the-pie-but-give-the-big-half-to-Obama decision in the health-care case.
And, finally, it's because running against (or for) the Supreme Court is difficult. Even in 1936, when everyone following the news understood that president Franklin Roosevelt and the Nine Old Men were in a death-duel, FDR chose not to foreground the issue as he cruised to electoral triumph. The general public doesn't deal with court issues well. If pushed, they basically think judges should be "fair," and that's about it.
It's a problem for political discourse, because the truth is that the Court, and the courts, are increasingly central to American politics. In the wake of Bush v. Gore, former Solicitor General and Whitewater independent counsel Kenneth Starr explained in a lucid book that the Court sees itself as the head of the central branch of American government -- "first among equals," as he phrased it, with Congress and the President assigned to follow the Court's direction. It's a startling change in Court attitudes.
Only a generation ago, justices were serious about deferring to Congress on matters of policy. And it is a change that has become more, not less, marked in the years since, as the political system has become more and more paralyzed by partisan rancor and pure stupidity. The "first among equals" view is on display in this year's just-started October term, in which the justices have plunged into a policy-making role on racial issues and voting rights, among other things.
That claim of centrality is increasingly being staked by lower courts as well, especially by the Court of Appeals for the District of Columbia Circuit. That court is the farm team that produced Chief Justice Roberts and Justices Scalia, Thomas, and Ginsburg. (Justice Elena Kagan was also nominated for the D.C. Circuit by Bill Clinton, but blocked by Republican senators.) It is also a kind of mini-Supreme Court for all complicated questions of federal regulation. (Years ago, as a judicial clerk on the Fourth Circuit, I described to a D.C. Circuit clerk a civil-rights case I was working on; she got almost teary-eyed, and said, "I decide cases about hydroelectric power rates.")
As the incomparable Steve Pearlstein recently described in the Washington Post, the Republicans on the D.C. Circuit have begun a slash-and-burn assault on federal regulatory power, drawing frankly on libertarian ideology rather than statutes or the Constitution. Republicans have blocked Obama's attempt to bring balance to that court; today's New York Times points out that, as of today, three vacancies remain unfilled despite the administration's efforts.
So judges matter to ordinary people. It's just nearly impossible to explain why. And in a town-hall-cum-slugfest debate, the nuances are liable to be wasted at best, and dangerous to the candidates at worst.