The best way to explain something to a person or group is to choose the example they could understand. Why didn't John used the words like 'look, the miracle working Jesus' or 'look, the one who will raise the dead' because non of this will suit the main objective of Jesus coming into this world that is to die for the sin of the world. What the people knew was the sacrifices that they have been doing from generations for the atonement of one's sin, the lamb they have been using for the same reason. 'Lamb' was very well related to the work that Jesus would do for the world. But this lamb was not just taking the sins of one person but it was ONE for ALL.