[Fundamentalist evangelical answer:] Because it says so in the Bible, aka Word of God.
Given that you'll watch them anyway, I wouldn't be so sure. Accepting for the sake of the argument that Hell is real and reasonably accurately described in the Bible, it's really, really horrible, with fires burning you forever, demons torturing you and immortal worms gnawing at your flesh for all time. Emphasis on the last bit: It's eternal, literally forever. One everlasting moment of about as much pain and horror as you can imagine, stretched out for infinite googolplexes of years of time.
You'd have to have a really active imagination to think up a fate worse than that. It makes the Hades of the Greeks look like a picnic. In this scenario, I'll forgive anyone who chooses Heaven, if God will let him in.
If you say this to evangelicals, they'll just quote some scripture that says God never changes. Like this one: