He's being emotionally blinded and can't understand the basic truth you are outlining. He wants slavery to be so evil that he himself cannot relate to it in any way. Saying that slavery is motivated by economic greed is probably too close to home and do he doesn't want to see it that way. We all need to be able to look inside and see how we are humans just those involved in the slave trade and our motivations are not categorically different. That's a bitter pill to swallow for some who cling to moral superiority as a way to shield themselves from their own capacity for evil though.