Pivoted left as he's taking a backstep? Show me any boxer that can hop straight back and throw a counter left and pivot out aswell. You're right, Conor could have pivoted out with a check hook but he chose to instead pull back and throw a left hand.
Also you have to remember that check hook only really work well when it's same stance, and also you need to think that check hooks are often thrown with the lead hand. When all the variables come together a check hook and pivot probably would have been a worse choice of punch in that sistuation.
Oh and btw...Pivot hooks are often used in MMA, it's just that a lot of people don't pick up on them.