Gorbunova and colleagues showed that it was not life expectancy, but body mass that regulated the expression of telomerase. Simply having more cells increases the likelihood that one will become cancerous. We humans, as large animals, would likely develop cancer much more often and much earlier if we didn't suppress our telomerase.
We haven't come across this anticancer mechanism before because it doesn't exist in the two species most often used for cancer research: mice and humans. Mice are short-lived and humans are large-bodied. But this mechanism appears to exist only in small, long-lived animals.