Because our default mental image of ancient Greek architecture is based on one, very conservative type of building: the temple.

From the seventh century BCE to the rise of Christianity, temples dedicated to the Greek gods were almost uniformly rectangular, built of stone, ringed by columns, and characterized by use of one of the two (and later three) systems of proportion and decor we call the Orders (Doric, Ionic, Corinthian). Not all cities could afford a full-scale temple - but those that could, built them.

Until the Hellenistic period, temples were the only real "prestige" buildings in the Greek world. Temples were the symbolic centers of the cities that built them - "home" of their patron gods, setting of their greatest festivals, and a reliable way of impressing rivals and visitors. The fact that temple construction was often an important part of inter-civic competition had the effect of disseminating architectural conventions - polis B would want to build a temple that looked like polis A's, but better. Polis C would try to top both, on so on.

But why did rectangular, colonnaded buildings with columns of certain types become the default mode for temples in the first place?

The answer seems to lie in the seventh century BCE, when the growing Greek cities of the Archaic period came into close contact with the major trading centers of the Near East, and large numbers of Greeks first saw the colossal temples of Syria and Egypt. These experiences may have sparked a desire to create more monumental religious buildings back home.

Up to this point, Greek temples had been wooden structures, based (most archaeologists think) on earlier domestic architecture. These early temples were simple buildings: rectangular, with mudbrick walls and a thatch roof, fronted by a porch with wooden columns. As Greeks who had seen the great sanctuaries of the Near East returned home and began to finance more impressive temple buildings, the conventions of the old wooden structures - the rectangular shape, peaked roof, and porch in front - were simply translated into stone.

The earliest stone temples followed the conventions of timber architecture very closely. For example, the triglyphs, the three vertical lines carved at regular intervals along a temple's frieze, are just stylized versions of wooden rafter ends. The first stone temples were also characterized by thick columns and a low profile; early Greek architects, unused to working with stone, wanted to be sure that their fancy new temples wouldn't come crashing down. These first temples, massively-built and sparsely decorated, were the origins of the Doric Order. Corinth, probably the richest city on the Greek mainland in the seventh century BCE, may have been the place where the Doric order coalesced; the city's tyrants had motive and opportunity to build lavishly. The first Temple of Apollo at Corinth (whose replacement, built a century later, still stands) may have been one of the first examples.

About a half-century later, among the prosperous cities of the eastern Aegean and Ionia (now the west coast of Turkey), another style of temple building emerged. Probably inspired by both the Doric temples of the Greek mainland and certain traditions of Near Eastern architecture, Ionic temples were characterized by more slender (and often fluted) columns, crowned by capitals with volutes (scrolls). The massive temples of Artemis at Ephesus and Hera at Samos, both built in the mid-sixth century BCE, seem to have been the first major representatives of the style.

By the late sixth century BCE, in short, there were two well-established sets of conventions for building Greek temples. Each quickly became dominant in one part of the Greek world (Doric on most of the Greek mainland, Ionic in the islands and Ionia), and both became well-established enough to effectively prevent the emergence of any new style (the Corinthian Order wouldn't become important until the Hellenistic period). Although the geographic boundaries between the orders gradually broke down (the Athenian Acropolis, for example, has both Doric and Ionic buildings), the joint dominance of Doric and Ionic continued for centuries.

Why, then, did these two similar-looking styles monopolize Greek temple construction for so long?

First and foremost, the inertia of prestige. Many iconic buildings - including, most famously, the Parthenon on the Athenian Acropolis - were constructed in the late Archaic and Classical periods. The renown of these structures, and their associations with the touchstones of Greek culture, was powerful enough to discourage architectural experimentation for a long time.

On a more basic level, innovation is almost never (at least in the premodern world) a strong suit of religious architecture. It is usually assumed that the kinds of buildings that have pleased the gods in the past will please them in the future - and that was certainly the case in the Greek world, where the basic forms of religious ritual (centered on animal sacrifice in front of a temple's open doors) changed remarkably little for many centuries.

On a more practical note, after the heyday of Greek temple construction in the Classical period, there was little demand for new temples in old Greece. Unless an earthquake or especially unruly sack destroyed a city's sanctuaries, both tradition and tight budgets militated against their replacement. In the territories conquered by Alexander, by contrast, there was (relatively speaking) scope for plenty of new construction and more innovative temple building.

Yet despite developments in the Hellenistic and Roman eras (above all, the advent of the Corinthian Order), the basic conventions of the Greek temple persisted, and were adapted by the Hellenistic kings and the Romans (initially via the Etruscans) in a wide array of contexts from Britain to Afghanistan. Even where the building itself was completely different from a traditional temple - as in the case of the Pantheon - a pediment was still slapped onto the front. Religion and culture had come to be inextricably linked with the conventions of Greek temple architecture.

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The best reference book on Greek architecture (though now rather dated) is A. W. Lawrence, Greek Architecture (5th ed.) (New Haven, 1996)