Why was the concept of pain beyond Jonas's comprehension?
The community in The Giver chose to gain "control of many things. But we had to let go of others" when it adopted Sameness. As memories were erased from the community's awareness, emotional and mental pain was forgotten. By the time that Jonas was selected as The Receiver of Memories, no one in the community except The Receiver had any concept of what it meant to be in pain due to memories of past events.
The list of rules Jonas received with his assignment specify that he is allowed to request and receive medicine for physical pain - caused by injury - only. As Jonas learned from The Giver, the memories he was to receive would cause great joy at times, but would also inflict tremendous physical and emotional pain.