Christianity. When Emperor Constantine converted Rome to Christianity, he began laying out various restrictions on the games including prohibiting it being used as a punishment or even as an option for criminals, forbade the branding of gladiators, and so forth. Emperor Honorius would then completely ban the games, which had already dwindled by then, in honor of the martyrdom of Saint Telemachus. [1]
[1] - https://en.wikipedia.org/wiki/Saint_Telemachus