Maybe they get tired of watching liars, thieves, cheaters, murders, bullies and molesters getting away with their crimes. If no one confronts the behavior of bad people it doesn't take long for the good people to be victimized or driven away. There are a lot more decent people than bad ones. I wish more people would show a bit more courage and stand up for their rights and "snitch". The bad guys should be afraid of being caught.
Sometimes for the safety of others or because not telling makes them feel guilty but some people also like to get you in trouble if they're jealous of you or don't know how to properly stick up for themselves