It's completely different. MMA tends to be more explosive which is why MMA fighters do more weights and explosive activities. Boxing is different which is why they do more roadwork and skipping. I've grappled with boxers and they get tired very quickly too. Because it's very different and they're not used to that type of cardio. The squeezing, pushing, pulling, sprawling type cardio feels different from the ability to constantly throw and block punches.
I've never had a boxing or muay thai fight, but i've participated in training camps with guys who have and have gotten very "Striking fit", and yet felt like my grappling endurance was shit. And at other times I've been very jiu jitsu fit and couldn't spar for more than a 3-4 rounds without getting spent. And obviously getting ready for both is different as well
Also, not to mention Floyd's style is a far more energy conservative style than Conor's. Using a wider stance and switching stances regularly does get you tired. Lastly, Conor doesn't have good cardio in MMA either. He's struggled every time he has had a longer fight.