Because there are so many evil people in the world. Usually evil is the product of selfishness and goodness the product of unselfishness. So, if you wish to promote goodness you should encourage people to be unselfish — as Jesus did.
Jesus talked about why there is evil in the world and people’s preference for evil when he said:
And this is the judgment: the light has come into the world, and people loved the darkness rather than the light because their deeds were evil.