So when discussing Gladitoral combat, the thing to remember is that it was a spectator sport, designed to entertain crowds of people in arenas. In Gladiatorial Combat: The Rules fo Engagement, Micheal Carter discusses the older view of gladiatorial combat as a homicidal encounter against the more modern view of it as a spectacle sport. He notes specifically that the epitaph of Urbicus which contains the line "and I advise that one should kill him whom one has conquered" (referring to the murder of a conquered opponent) was the exception to most other gladiators epitaph's, most assigned death "to the Fates" and boasting that they "did not hurt anyone". Here it is clear that gladiator's weren't usually attempting to kill one another, at least in the more standardized, regulated sport.

On top of fighting for spectacle rather than murder, there existed officials that watched over the matches. The most important to our answer here is the Summa Rudis, noted to be a technical expert who determined when a contest should be stopped, generally prior to the fatal blow being struck. Here it is clear that armour didn't need to be worn as bulkily because of the fact that murder was not the primary objective of the gladiators, it was to win the fight, and the existence of the Summa Rudis exemplifies that.

Prehaps the more pertinent answer to this question however is the standardization of Gladiator 'types'. Some of the most well-known types tend to be depecited with little to no armour, such as the Retarius (wearing little to no armour below the chest), and the *Thraex/Thraeces (*wore a helmet and armoured greaves), though both the Murmillo and the Samnite were heavily armored for gladiators (each wearing heavy helmets, and the Murmillo having their distinctive armoured sleeve). Each class had specific rules on both what weapons the gladiator could use, as well as the armor they could wear, with the standardization going so far as to have specific styles that gladiators could use (Carter even notes that spectators would complain of gladiators being too textbook).

Gladiators wearing little armour comes down to both the spectacle of the sport and the types of gladiators that existed. While no gladiator would be as armoured as your basic Roman soldier, this was due to the context they fought in, one in which the spectacle of the fight was the goal, and as such maneuverability and finesse was favoured over heavy armour designed to take killing blows. Overall, the combination of a lack of intent to murder and rules which gladiators followed are why they wore comparatively little armour.

Sources Used:

CARTER, M. (2018), ARMORUM STUDIUM: GLADIATORIAL TRAINING AND THE GLADIATORIAL LUDUS. Bulletin of the Institute of Classical Studies, 61: 119-131.

Carter, M. "Gladiatorial Combat: The Rules of Engagement." The Classical Journal 102, no. 2 (2006): 97-114.

Reid, Heather L, (2006) Was the Roman Gladiator an Athlete?, Journal of the Philosophy of Sport, 33:1, 37-49.