#4 He doesn't endear himself to his detractors
Each wrestler has a certain way about them, whether it's how they walk, how they talk or the mannerisms they show. There is something about them that fans either like or hate. In the case of Roman Reigns, there just appears to be something about him that gets a specific demographic irate.
Reigns walks to the ring with a sense of confidence about him, which some may misconstrue and believe to be arrogance.
Contrast that to how Cena always plays to the crowd during his entrance, whether he is being cheered or booed. When Reigns faced off against Brock Lesnar at Wrestlemania and was smiling after each punch, kick, and F5 he took, it generated more boos for him than cheers. We don't know for sure what the actual intentions of his mannerisms may be, but he may simply be showing that he is willing to take someone's best shot and keep going.