Fitzgerald is generally known for his lush, flowery, expansive prose – check out any passage in The Great Gatsby and you'll see exactly what we're talking about. The man doesn't hold back. But Gatsby was published in 1925, and Fitzgerald is writing "Babylon Revisited" give years later with what an intensely sobered attitude. The lush, extravagant days of the 1920s have passed, and it's time to face reality – and prose – with a more solemn attitude. Let's compare a Gatsby passage to a "Babylon" passage, both describing a nightlife scene:
The lights grow brighter as the earth lurches away from the sun, and now the orchestra is playing yellow cocktail music, and the opera of voices pitches a key higher. Laughter is easier minute by minute, spilled with prodigality, tipped out at a cheerful word. The groups change more swiftly, swell with new arrivals, dissolve and form in the same breath; already there are wanderers, confident girls who weave here and there among the stouter and more stable, become for a sharp, joyous moment the centre of a group, and then, excited with triumph, glide on through the sea-change of faces and voices and color under the constantly changing light. (The Great Gatsby, Chapter 3)
After an hour he left and strolled toward Montmartre, up the Rue Pigalle into the Place Blanche. The rain had stopped and there were a few people in evening clothes disembarking from taxis in front of cabarets, and cocottes prowling singly or in pairs, and many N****es. He passed a lighted door from which issued music, and stopped with the sense of familiarity; it was Bricktop's, where he had parted with so many hours and so much money. A few doors farther on he found another ancient rendezvous and incautiously put his head inside. Immediately an eager orchestra burst into sound, a pair of professional dancers leaped to their feet and a maître d'hôtel swooped toward him, crying, "Crowd just arriving, sir!" But he withdrew quickly. ("Babylon Revisited," 1.53)
Clearly the author's attitude is different, but try to look at these again at the level of the prose only, even ignoring the content. Check out things like sentence structure (intricate vs. straightforward), sentence length (long vs. short), even word choice. See how the clipped and grim final sentence of the "Babylon" passage would be out of place in the Gatsby paragraph?
Of course, Fitzgerald is still Fitzgerald, and we'll admit that there is the occasional Gatsby-esque prose moment to be found in "Babylon" (not the least of which is Charlie's sentiment that the money he spent was "an offering to destiny that he might not remember the things most worth remembering, the things that now he would always remember" [1.59]). Our point is that such exceptions are just that – exceptions.