What limits the number of quick punches a boxer is able to throw in a combo (eg. a 1-2-1-2... or 1-2-3-2... combo)? Is it related to breathing and cardiovascular ability, muscular strength, weight or something else?
I'm not necessarily referring to a combat situation, where other factors (the opponent for example) will have an influence. Even while punching a bag or pads, or even shadowboxing, some people seem comfortable throwing 15-20 punch combos, while others struggle with more than 4.
EDIT : (1) The punches in question are full-effort, with proper form, how they would (should?) be in an actual fight.
(2) '15-20 punch combos' may be misleading, I actually mean a string of combos without 're-setting'. A simple example being 1-2-1-2-1-2-1-2-1-2-1-2-1-2-1-2-1-2-1-2, (1-jab, 2-cross) which is the ending of this workout.
While experience, conditioning, practice etc. will surely play a role, that is not what I'm looking for here; I'm trying to understand what specific body factors make one stop (and therefore, need to be worked upon).
Background: Boxing novice, no real-life combat experience, not overweight, distance-runner.