Why did the LORD put a mark on Cain?

Answer: So no one would kill him

Cain feared being killed, so the LORD placed a mark on him "so that no one who found him would kill him."

“Behold, this day You have driven me from the face of the earth, and from Your face I will be hidden; I will be a fugitive and a wanderer on the earth, and whoever finds me will kill me.””— Genesis 4:14 (BSB)