Reddit mentions of The Power of Images in the Age of Augustus (Thomas Spencer Jerome Lectures)

Equating quality and realism is actually quite problematic. Modern works of art normally don't stand out by how lifelike they are either. But that doesn't mean that there has been a dramatic decline in artstic quality since the 19th century.

In fact I would say roughly the same amount of skill went into producing the portrait of Augustus and of Constantine. Realism on the other hand isn't really the right word to describe any one of those two. The statue of Augustus is highly idealized and more influenced by standards of beauty inherited from classical Greece than by the emperor's actual appearance. Portraits of the man looking like this were still produced when he was more than sixty years old after all. By imitating the style of the Classical period the artist could showcase the emperor's conservative values, his decorum and his auctoritas.^1
Now Constantine is shown to be a larger than life figure with an unblinking gaze fixed on the horizon. The emperors of Late Antiquity were portrayed as charismatic rulers that were totally different from any mortal man.

On the hole it is true that Late Antique art often rejected the aesthetics that had been established in earlier ages and by doing so could get quite abstract or even comical at times. Even so those older aesthetics were not totally forgotten. The same goes for medieval art. Only with the beginning of the Renaissance Classical Greek art was again seen as the single gold standard to imitate.

Why it came to such a dramatic stylistic change is not easy to answer. Late Antiquity was in many ways a time of cultural upheaval. Rome ceased to be the centre of the empire and its old elites had to contend with new men coming from the provinces. Christianity was on the rise. After centuries maybe now Roman artists were ready to try something radically new.

1. Paul Zanker, The Power of Images in the Age of Augustus(1990)

Well, here's one :

The popular book I, Claudius, and the BBC series based on it, posit that Livia, the wife of first Roman "emperor" Augustus, killed off a series of Augustus's friends and closer family members to ensure that her son from a previous marriage, Tiberius, would be the only choice for Augustus's heir.

Of course there is no evidence for any such conspiracy. The only evidence for it at all, at least that I'm aware of, is in the form of "motive." And that is, the Southern Frieze of the Ara Pacis ("Altar of the Augustan Peace") depicts Augustus's friend and trustworthy general, Agrippa, in a prominent place relative to Augustus's family in a religious procession.

Some academics have speculated -- I think Zanker in a book on Augustan art -- that this implied that, at one time, Augustus may have considered Agrippa, the architect of his victory over Antony, his natural successor.

If Livia expected her child to succeed instead, that may have provided motive. But this is scant evidence indeed, however interesting .