The simple fact is that punishment deferred too long has little effect on people's behavior. If they got a taste of hellfire every time they sinned, they'd be the most subservient to God people on earth; but the punishment isn't now or tomorrow, it's after they're dead; and they get multiple chances to repent and change their ways; it doesn't seem urgent enough to prevent them from rationalize doing what they want to do anyway.
I'm not anti-Christian. I'm anti-stupid.