Modernist architecture’s rejection of ornament is understandable. The Modernists had no idea what any of it meant. So it seemed stupid to them. So they excised it.
Ditto for all traditional forms. These were all meaningless to the Modernists. So they rejected them, root and branch. In this, they were aided by developments in the economics and technology of building. But the impoverishment of modern architecture was spiritual before it was material.
*Everything* is spiritual before it is material.
In traditional architecture, ornaments all signify. They all have meanings, and those meanings all terminate ultimately (albeit generally by way of a chain of connotations and denotations) upon the Ultimate and our proper relation to him.
The sacred terminus ad quem of traditional architectural ornament is clear enough in temples and churches (albeit arcane to most moderns, alas: they vaguely feel that the trifolium, the rose window, the vault, the Mogen David are sublimely sacred, but they have no idea why they are sacred in plain concrete fact, for they have no notion of what these ornaments mean). When it comes to the ornamental traditions of neo-classical architecture, or to the form of traditional houses, it is less obvious. Yet as with so many things we take for granted in “secular” culture (as sport, drama, warfare, the hunt, agriculture, etc. – even, indeed, rank commerce and politics), the former derive all from the ancient rites of sacrifice, old already in the Bronze Age; while the latter echoes the notion that houses and their households are domestic temples, each best and most properly arrayed so as to accommodate a yearly round of homely sacred ritual – indeed, for millennia, the custom was to bury the honored ancestors in the floor of the house, and to invoke their presence at each meal (as saints and nobility are buried in churches, and all the holy dead partake of the one eucharistic feast).
Likewise also for ancient roads, and for the plans of cities: their prominent features were traditionally oriented to celestial phenomena. For example, the gates of old cities (so often twelve in number) often point to the gates of the rising or setting sun at various seasons. Each gate was a jewel, a tribe, an angel: as the Essene Gate in ancient Jerusalem.
In traditional cultures, all building was sacred. The mason, the carpenter, the architect, the surveyor, the gardener: these were all sacred offices.
[In traditional cultures, of course, there are no merely profane offices, for in coherent cultures ordered by and to the Logos, everything not essentially ordered to evil partakes of the sacred, at least a bit.]
The Modernist architects forgot the signification of architectural ornaments, or never learnt it. This might have been due to a simple failure of pedagogy, or to their own uncomprehending puerile rejection of the category of the supernatural, to which all ornament even now refers. Or both. More likely the latter, actually: for, what can the supernaturally intended teaching of a traditional master of the building arts seem to his student who has rejected supernature, other than the most arrant nonsense?
Ornaments became then to the Modernists meaningless, empty, superfluous, expensive, indeed somewhat disgusting adulterations of the pure austere utility that was the only meaning or signification their materialism was competent to understand.
It made perfect sense, given the terms available to them. It has not worked out well.
Post-modernism in architecture is a reaction to the Modernist rejection of architectural meaning, and thus of any architectural language. It wants to re-introduce the vernacular meanings of traditional architectural forms – including ornament – if only for purely utilitarian reasons: namely, that people (including the architects themselves) simply like the result much better than the stark banality of the International Style, and find it more comfortable.
But as you may have noticed, post-modern ornament never quite works. It fails to close the deal. Post-modern buildings are more pleasant than their modernist precursors, to be sure. But their gesture at meaning is a simulacrum only; it is fake. It is uncomprehending, artful enough but stulted, dead, ineluctably meaningless despite its earnest noble intentions. It is akin to the evolutions of cargo cults, who did not understand what airplanes or white men were.
A successfully humane and truly beautiful built environment awaits the rediscovery by architects of the sacred language of building, implicitly also then of all material nature, and of the consequent hieratic aspect of their own profession – and of their duty thereto.