Because they have turned away from God.
From what I have seen and heard, there are a thousand and one reasons why you can be sent to Hell. Most of them you aren't told about until it is too late. If you choose to believe in this with nothing to support it, that is your choice. If you look at all the loop holes, you simply cannot win at this game. From what I hear, you can live a perfect life, never hurt or harm anyone in any way, do everything letter perfect and if you don't bow down to JZ or take His name in vain, your done for. No one can manage to be that good all the time.