Bill, it has to do with the ramp. Think of it as a pitched roof on a house. the lifter has to ride up one side and down the other as the valve opens and closes. If you want a high lift (lobe large) then if the duration is too short (does not look like /\ but ||) the lifter cannot ride up the ramp of the lobe correctly and you will lget cam and or lifter failure very quickly. You can go a little more lift and less duration with a roller setup but not much.
Larry