It's because of the appeals process. In the US, the death penalty cannot be administered until the defendant has either declined to appeal further, or completely exhausted the appeals process. The legal process in the US is intensely expensive, and nobody facing the death penalty ever gets their sentence and just says, "Well, guess that's that, I'm convicted, why bother appealing?" The appeals process takes years, and thus adds years to their life expectancy. And personally, I'm glad the appeals system is so intense; I'm opposed to the death penalty to begin with – but if we're going to have it, I at least want to make sure that he have given people every reasonable chance we can to prove innocence or mitigating factors.
While it is true that religious sentiment in the US correlates with support for the death penalty (which is regrettable), within religious communities, amount of time spent in religious service (either laity or in vocational ministry), as well as knowledge of church doctrine, correlates negatively with support for the death penalty. Also, Christian support for the death penalty, thankfully, has declined somewhat in recent years.
i.e. people of a certain disposition tend to be religious and also tend to favor the death penalty. But heavy involvement in religion or in-depth exposure to doctrine tends to mitigate that effect, though not necessarily completely eliminate it.
Proof please, let's not just bring these 'facts' to air without supporting evidence.
