Because Benavidez is a natural cruiserweight if not a heavyweight. It's not fair to match a 6'2'' giant with a 5'7'' super middleweight.
No old school boxing fan would really believe that nonsense. Its weight not height divisions. Callum Smith was taller than Benavidez anyway.
The momentum is bigger when you're taller and have longer limbs. And they're same weight on weigh-in day, not on fight day.