Dahlia Lithwick argues that popular support for the death penalty has declined in recent years from very widespread support to merely substantial majority support (67%), but because the Supreme Court has not declared it unconstitutional in view of this decline, the Court is at odds with the views of the people. Well, she doesn't say it quite like that. Instead, she hedges a bit, and uses phrases like "there now seems to be a subtle hardening in favor of the death penalty" on the court. But to me the implication is that she thinks they should say it's unconstitutional.
So, first off, what on earth is she thinking saying that the Court is not following the people when 67% of people support it? Granted, support has declined, but it's still very high, probably higher than among the Justicies.
But more importantly, there's a big difference between people having doubts about the death penalty as a policy matter and it being unconstitutional. It can be a horrible policy but still be constitutional (or vice versa). There's not always going to be a connection between the two. The Court's job is to address constitutional issues, not make policy, right? I guess for some people these issues are one and the same, which is something that is depressing to be reminded of.
The Volokh conspirators dissect the issue here and here.