World Conquerors
More or less art stayed the same while the Romans conquered the world and founded their own empire. Though their most outstanding achievement was that of civil engineering. One of the differences that started to emerge between the Hellenistic art and the Roman period is how the emperors allowed artists to make them "more lifelike and uncomplimentary than the Greeks had attempted." When we see the busts of the emperors we don't see Hellenistic beauty, we see more lifelike features without seeming trivial.
Talking about lifelike features, the Romans incentivized the art that showed their power as empire. Stories are told in their art about victories and how they dominated their opponents. This accurate rendering of the details changed the character of art. When Christianity entered into the picture, a combination of telling stories as simple and as clear as possible got together with the pagan Hellenistic art. People started to idealize and imagine how god looks like, Christ is shown with youthful beauty and the stories from the bible are portrayed as simple and clear as possible. Besides, the time to craft sculptures in the Hellenistic manner became scarce in the middle of wars, revolts and invasions. More "rough-and-ready methods" emerged like the marking of the features of faces and bodies. Art was back to being religious.