It's just like rooster fighting or pitbull fighting. Bodytype.
Buff and stocky is fast but short distance street fighter. Such as Mike Tyson.
Lean and mean is long distance boxer. Such as Larry Holmes.
Boxer beats a street fighter 65% of the time. Street fighter beats boxer only 35% of the time. If both are natural and prime and great.
Tyson and De La Hoya at times neglected training but some fighters just have the ability to be relaxed when boxing and thus seemingly never got that tired. Rocky Marciano had some amazing stamina some claim he must of been blood doping, but he trained like hell and due to his dedication to training could start and maintain a pace that few if any could keep.
Fighters have good stamina . You improve with training but even a person with good stamina can get tired as getting punched will screw with your breathing which hurts.
It's just because their training doesn't involve stamina training because (although it would help a lot) isn't considered necessary. Although boxers and have bad stamina it is necessary to have good stamina for martial artist.
Of course, it's lack of or neglect of training, especially roadwork. I mean, running is your number one stamina builder.
is it because they dont train enough? or is it something u have naturaly and can only improve slightly with training?like for exemple oscar de la hoya and mike tyson are known for not having great stamina