People would get on stage and they would either mess up their performance or fall down and get injured. Or similar bad things would happen.
The actor would come off stage when the show was over and accuse their friends of putting a curse on them by saying "Good luck". They would be really mad at their friends for putting that curse on them with their kind words.
So, people started saying just the opposite. Rather than wishing them good luck and risk cursing them and making them fail, they wished them bad luck, by telling them to "Break a leg".
The reason was, because, when they said: Good luck, it often had the opposite affect. Then the performer was mad at them and said it was their fault.
People figured that by wishing them bad luck, that will hopefully have the opposite affect to. So, the performer will not blame them for causing them to fail.
This was a really long time ago when people started to do this. The tradition never died. People have been doing this ever since. Until now, almost nobody knew why!
