I suppose there are all kinds of potential reasons, but I believe it's because God had been speaking to man for thousands of years. Through direct communication and Mosaic law, God told us what He wanted us to do. Of course, we didn't do it. It's painfully evident throughout the Old Testament. We disobey God.
Jesus was God's final solution to this problem. Even though man disobeyed, God Himself provided dispensation. His name is Jesus. Jesus didn't need to restate the law. He interpreted it. He told us what we really needed to know.
There were all kinds of commandments in Jesus' day. You're familiar with the "Ten Commandments," but you may be less familiar with the other laws, such as those described in Leviticus. There were hundreds and hundreds of them.
Jesus boiled it all down for a confused and misled people.
Which law did Jesus say was the most important? Love God. The second most important was to love man. In Matthew, he goes on to elaborate that we are to treat other people the way that we want to be treated.
It's easier said than done. But if our goal is to store up treasure in Heaven, then we must do what Jesus tells us to do. It goes against our human nature. It goes against every instinct I have in this world. My instinct is to take care of me and mine. I don't have a lot left for others after that.
And yet, Jesus tells me to do it. Following Christ means following his instruction.
Do unto others. Not "before they do it to you." Not "it's payback time." Not "but they did it first."
Simply, do unto others what you want them to do to you.