"The result shows that gladiators mostly ate a vegetarian diet. There is virtually no difference in terms of nutrition from the local "normal population." Meals consisted primarily of grain and meat-free meals. The word "barley eater" relates in this case to the fact that gladiators were probably given grain of an inferior quality."
1) You don't waste expensive food on people who are about to die stabbing each other to death for your entertainment.
2) You don't even bother to waste good grain on them.
3) Most people ate a mostly vegetarian diet, meat is expensive and hard to transport.