Because the Serpent tempted Adam and Eve and they sinned by eating the fruit that God told them not to eat, God said to the Serpent in Genesis 3:14 "Cursed are you above all livestock and all wild animals! You will crawl on your belly and you will eat dust all the days of your life. And I will put enmity between you and the woman, and between your offspring and hers. he will crush your head, And you will strike his heel." This is the first promise of a Savior and was fulfilled by Jesus) 

Not only was the Serpent cursed but there was... 

The Cure... 

For God so loved the world that He gave His Only Begotten Son that whoever believes in Him should not parish but have everlasting life. John 3:16

Jesus crushed Satan by His suffering, death and resurrection on the cross.