he had already explained why. in chapter 13
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The people need a receiver of memory to contain the memories, hold the pain, and store the knowledge.
The Giver/ Chapter 13/ Page 104
The receiver is a human super-computer. He can take all the factual data and make decisions for the best of the community. Unfortunately, he is treated just like a computer-a tool for the elders to use. They don't care how he feels.
The people need a receiver of memories to hold memories so the people don't know about past life.