Hypothesis: Religious people (or at least Jews and Christians, which are the religions I'm most familiar with) tend to say that life and death are ultimately in the hands of God/G-d. I suspect this is a way of avoiding survivor's guilt, though both groups are generally in favor of medicine.

From memory: a news story about a conference on medical ethics where the Orthodox Jews were the only ones in favor of life extension.

I suspect that any religion with a vividly imagined heaven has to have rules against suicide, or else the religion won't survive. It's plausible to me that the revulsion against life extension is a mere side effect of the rule against suicide.