Because God's ways are not well understood?
Matt. 25:41 “Then he will say to those on his left, ‘Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels. 42 For I was hungry and you gave me nothing to eat, I was thirsty and you gave me nothing to drink, 43 I was a stranger and you did not invite me in, I needed clothes and you did not clothe me, I was sick and in prison and you did not look after me.’ 44 “They also will answer, ‘Lord, when did we see you hungry or thirsty or a stranger or needing clothes or sick or in prison, and did not help you?’ 45 “He will reply, ‘Truly I tell you, whatever you did not do for one of the least of these, you did not do for me.’ 46 “Then they will go away to eternal punishment, but the righteous to eternal life.”
So, it appears some people will be surprised by the eternal punishment that awaits them. There is another passage, where Jesus tells those who did miracles in his name that he never knew them.
It's not uncommon for Christians (especially those with anxiety issues) to suffer from the fear of Hell. It's not like God is clearly stating who is in and who is out.
As to the Bible teachings on hell, here is a Christian apologetic site discussing Hell