Please explain why is prostate the most deadly cancer?

Wrong: Prostate is not "the most deadly cancer". Actually, which cancer would be called "deadliest" is a matter of debate, depends on the circumstances. Prostate cancer is very common and is often found incidentally in elderly men who die of other circumstances, meaning it was not growing and did not cause death. There are aggressive ones, no doubt, but as a rule i would not call it "the deadliest".