We don't really know
We commonly speak of the Classical Greco-Roman civilization as if it was uniform. And in many ways, it was; ideas travelled freely, artists (and works of art) travelled freely, and they sort of look similar. But they are not the same; it is a common observation that in the Greek-speaking east, art tended to idealize the subjects, whereas the Latin-speaking west it tended to show them as individual humans.
Two great generals. On the left, an Ancient Greek herm of Themistocles. On the right, a 1st century BCE Roman bust of Scipio Africanus. Note the vastly different treatment; the Greeks showed the Greek general with ideal manly traits, exuding an air of serenity and calm determination; the Romans showed the Roman general as a real flawed man, pursing his lips in an attitude of stubborn obstinacy. Pictures from Wikimedia.
Two great philosophers. On the left, the Greek Aristotle; Roman copy in marble of a Greek bronze by Lyssipus. On the right, the Roman Seneca. Compare the god-like father of logic with the fat, old, wrinkled Roman ethicist. Pictures from Wikimedia.
Two great statesmen. On the left, the Athenian Pericles; Roman copy of a Greek original. On the right, the one and only C. Julius Caesar. Note how the Athenian demagogue is shown in military glory, whereas the Roman conqueror is shown as a civilian and with asymmetric traits. Pictures from Wikimedia.
In general, it is widely known that Roman portaiture was intensely realistic -- they did actually show the warts on the faces of great men, they showed them wrinkled and fat and bald! Now, the Romans did actually like Greek statues, very much, and paid handsomely for faithful copies of Greek statues. We have many such copies; so we know that Roman sculptors could imitate the serene ideal figures favored in Greek statuary. But for their own domestic productions, they didn't; even when creating artworks for blatant propaganda they did not completely tranform their subjects into divine beings. Compare the treatment of Alexander the Great and Trajan:
Two propagandistic statues of emperors. On the left, Alexander the Great. On the right, Trajan. Alexander is shown as a demigod, his humanity almost completely effaced. Trajan is a ruler, shown with his own less than perfect likeness. Pictures from Wikimedia.
Trite explanations
The easiest explanation is to lean on the "national character". The idealistic, poetically minded Greeks idealized their subjects, the pragmatic self-confident Romans tended to show the individuality of each subject. If used for anything more than fluff, this runs into deep problems, not the least of which is that it is almost racist. (And anyway, in the antiquity it was the Greeks who were perceived as sharp negotiators who eagerly resorted to shady business practices.) The musically inclined easy living Italians liked to show their subjects as beautiful and carefree, whereas the hard working Flemish and German artists put a lot of effort in the details and found joy in depicting the reality of life. And so on.
Another easy explanation is to resort to the "spirit of the times". In tranquil times art tends more towards idealism, in more turbulent times realist individuality prevails. Except that this bunk; Pieter Bruegel the Elder (died 1569) and Michelangelo (died 1564) were contemporaries (although the Dutchman was much younger):
Two artists active in different countries during first half of the 16th century, a.k.a. High Renaissance. On the left, a detail from Pieter Bruegel the Elder's Nederlandish Proverbs; on the right, an ignudo from Michelangelo's magnificent Sistine Chapel ceiling. Pictures from Wikimedia.
Ah, and whose style is more realistic? Is it Michelangelo's, whose figures are always anatomically correct, albeit idealized, or is it Bruegel's, whose composition and attitudes are definitely faithfully exaggerated slices of reality, but whose figures are almost always somewhat cartoonish? What is realism, really?
For an even more striking example, consider the Pre-raphaelite movement which flourished in the United Kingdom in the second half of the 19th century. One would be hard pressed to find any little hint that those paintings were created while the country was busy with the second industrial revolution and was putting a lot of effort into conquering half the world.
The Hireling Shepherd, by William Holman Hunt. This wholesome no fat no sugar painting was created in 1851. What British Empire? What industrial revolution? What Australian gold rush? Look see a handsome English shepherd putting the moves on a rather doubtful beautiful and barefoot English shepherdess. Don't forget to note the cute lamb. Picture from Wikimedia.
Real explanation
The real explanation is that this is simply how it is; artistic styles are invented at some time and place, are widely imitated, and then they eventually evolve into a new style or go out of fashion. Thomas of Aquino nailed it when he gave his famous definition of beauty: "pulchra sunt quae visa placent", beauty is what is pleasant to look at. There really is nothing more. Some artist invents something new; some people find it pleasant to look at; other artists copy the technique or the style; lots of art is made in that style; and then some other artist invents something new, or revives something old, and some people find it pleasant to look at...
As a secondary aspect, let's not forget the power of entrenched interests. Once a particular style begins to be taught in schools, it gathers a lot of momentum from the sheer mass of vested interests. 19th century European painting was dominated by the trans-national (and mostly lifeless) academic style because that was what Academies of Art taught, that was what was shown, that was what received prizes, that was what was commissioned by bureaucrats to adorn their bureaucratic dens of state. Remember the scandal of the Salon des Refusés of 1863, where Gustave Courbet, Édouard Manet, and Camille Pissarro exhibited their decidely non-academic works! (Including the boundary-breaking Déjeuner sur l'herbe, which the prudish official Salon had refused as immoral despite its impeccable pedigree.)
All motivations are individual (or economic)
Most people like some figurative art and some abstract art. Nobody likes abstract art exclusively, nobody likes idealistic figurative art exclusively, and nobody likes realistic figurative art exclusively. Isn't it possible to like Vivaldi, and Bach, and Rammstein and Laibach? It surely is. Isn't it possible to find pleasure in looking at paintings by old Flemish masters and at paintings by Mondrian and Kadinsky? It surely is.
So how does a style become dominant in a culture, be it limited in time and space?
That is what is taught. Most artists don't go out of their way to invent a new technique or a new style; they must make a living, and they do what they were taught to do.
That is what is expected. I mentioned European academism above; I can add the glorious socialist realism which ruled over a wide territory for more than half a century because that was what the bureaucrats wanted.
That is what other people buy. One should never ignore the power of social conformism. If one's peers favor a certain kind of art, one commonly tends to favor it too, whether it corresponds to their inner wishes or not.
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