Boxers/Kickboxers have better defense because it's easier for them.
In MMA the gloves are smaller. They slip through guards easier than in bigger gloves. If boxers were using MMA gloves they'd be getting more KOs for sure.
And boxers only have to worry about punches. Yes, there's lots of ways punches can be setup, but in MMA you have to worry about all that, the smaller gloves getting through, and you have to worry about kicks, elbows, clinch attacks, attacks to the legs, and takedowns.
This is why there's so much more range management in MMA. None of these MMA fighters want to stay at the range boxers and Kickboxers do because they know they're going to have a way harder time of it and are gonna get caught way easier.