This question is based on the false assumption that architectural and fashion styles in the ancient world lasted a long time. They did not. Among the elite in particular, the particular styles of hair, clothes and shoes that were in vogue changed constantly, and both men and women of means were obsessed with staying fashionable. The most extreme example of this is that it is possible for modern historians to date the portrait busts of Roman ladies by the way they did their hair. No self-respecting noblewoman would be seen sporting last decade's haircut.

In architecture, too, styles were always shifting, and the trained eye will instantly be able to tell a Roman temple from a Greek one, or a Hellenistic from a Classical Greek one. New forms of column design, facade decoration and statuary were constantly being developed. Even someone with a casual interest in ancient art will be able to see at a glance whether a Greek statue was made in the 6th, 5th or 4th century BC.

If basic architectural designs did not develop as quickly or as radically as they have in modern times, it is primarily because we are building so much more than the ancients ever could, and our construction technology is changing rapidly. In ancient times, the level of available technology simply didn't give architects as many options as they have now. But that certainly didn't stop them from trying.

We interact with so many more people and cultures today. 20th C aesthetic values prized ahistoricism, uniqueness, individualism, etc. Materials, manufacturing, and construction techniques are so much different-iron and steel, mass production, space age polymers, machines, etc. From a different perspective, they were changing in the past, but the general public is not educated enough to catch on to the nuanced differences between, say, the classical and hellenistic styles in Greek architecture and sculpture.