Why? There are theories galore -- a belief in retribution, exaggerated perceptions of the risk of being a victim, suppressed racism, etc. A new favorite is that, unlike the Europeans, Americans aren't brainwashed by their chardonnay-sipping, pacifist-minded elites. But the only one I'm qualified to comment on -- and the only one that I'm sure is not grounded on solid evidence -- is that capital punishment is a deterrent to murder.
I got interested in the subject in the 1970s, when the Supreme Court was still liberal enough to entertain the idea that capital punishment might be unconstitutional. This was also an era in which economics was especially entrepreneurial, and economists were cockily opining on everything from slavery to drug addiction to marriage. Defenders of the death penalty got hold of a paper by one of them (a grad student at Chicago named Isaac Ehrlich), who used an econometric model to demonstrate that each execution saved multiple innocent lives.
A bunch of other economists (including me) heaped criticism on the research in scholarly journals. But what filtered through to the Supreme Court and other opinion makers was that "the experts disagree." And though the passion has died down, it occasionally still flares. As recently as 2003, a couple of economists reported that one lethal injection reduced the murder count by 18. Not surprisingly, their statistical work was subsequently dumped on by other economists.
I hope (and I'm pretty sure) that the death penalty adds little deterrence to that created by the prospect of life in one of the hellholes we call prisons. But putting on my former professor's cap, probably the best one can say is that, if capital punishment is indeed a deterrent, the signal is way too weak to be distinguished from the noise.
Why bring this up now? With the pounding that the hard sciences are taking these days on subjects ranging from climate change to evolution to the value of vaccines, it would probably be prudent for economists to keep their heads down. It's hard enough to listen to what people are saying about Keynes.