You’d be forgiven to assume that they got collective amnesia.
For a while there, art looked pretty good. At the height of Greek and Roman civilization, artists could reproduce the human body with photographic accuracy. And from solid marble no less. Between 500 BC to 300 AD various styles dominated — the serene statues of Praxiteles, the flamboyant drama of the Hellenistic statues, and the unvarnished veracity of the Roman busts. In all these approaches art moved beyond its traditional use as a tool of ritual or propaganda. Art became a distillation of societies’ highest values — truth or beauty or all that — and artists eagerly pushed skills and techniques to better embodies those lofty ideals.
And then, in 293 AD, seemingly out of nowhere, things start to look like this:

Suddenly, contrapposto has been thrown out the window along with proportion, naturalism, and generally the Greco-Roman aesthetic sensibility. They’ve started using purple rock for god’s sake.
This shift in style has birthed a thousand theories.
The stock answer, long trotted out in Art History textbooks, was that the knowledge of the Greeks and Roman was lost. There was a regression in artistic skill and ability. The sculptors of Western civilization got collective amnesia and couldn’t remember their first names let alone how to carve realistic bodies out of marble.
Another group suggests that Christianity drove out the classical aesthetic. As a monotheistic religion, getting rid of pagan gods and idols was a top priority. Even if the statues of pagan gods and idols were really pretty. But, iconoclasm in the Byzantine Empire didn’t start in earnest until the 700 AD. And Christianity didn’t become the state religion until 380 AD. These trends are a bit late to explain why sculptures on the Italian peninsula in started to look like legos around 300 BC.
The shift in style does coincide with change in the form of government. As the empire became too cumbersome to rule, leadership was divided up between four tetrarchs (otherwise known as the blocky figures in purple). This system of government hardly provided the stability it promised and Roman civilization continued to spiral out of control. As the Roman empire went through an identity crisis, so did the art of the time.
The so called “decline of form” can thus be read not as a loss of knowledge or backlash against icons but a conscious attempt to create a new style to parallel a new age in Rome. Though the political situation and art appears rather discordant to modern eyes. It illustrates the danger of oversimplifying art history into a neat linear narrative. What we might see as regression can very easily be reinvention, revision or revolution.