He didn't tell them about the snake because the snake wasn't the thing that was a threat to them, it was themselves. They could have rejected the snakes temptations, but they didn't. They consciously made a choice to eat from the tree, knowing the consequences. In Genesis 2-16 God says "You are free to eat from any tree in this garden; but you must not eat from the tree of the knowledge of good and evil, for when you eat from it, you will surely die." God gave them all the information they needed. Telling them about Satan would have revoked their free will in the situation, which was a gift that he gave to them, and he would have given them the knowledge of evil which they would have received if they ate from the tree.