PS - I didn't say Paul, Jesus, and John didn't teach about Jesus' death. Look closer at what I did write:

None of the people you named above clearly explain why Jesus had to die. They simply say He suffered and died with our sins on the cross. The SOP is the only [inspired] place where God has clearly explained why Jesus had to die.

I added "inspired" here because that is what the context was referring to. I believe other people have written about it.