Why don’t we build in classical style anymore?

the simple answer is cost and technology versus tradition (and/or style). As an answer, each of those factors works hand in hand to shape why we don’t create buildings like we used to. The primary factor is cost, and especially skilled labor.

As the cost of skilled labor has gone up in proportion to the overall budget of buildings, the high labor aspects of buildings such as stone work and ornamentation have become more of a challenge to use. Architectural design theory rejected non-functional ornamentation as made famous by Aldof Loos and his lecture “Ornament and crime” where he attacks ornamentation without purpose. As modernism evolved, especially with the Bauhaus school, (Walter Gropius, Le Corbusier and friends) became the new language of the evolving technology (high strength steel).

The style of buildings has become less ornamented as our theory of why we use ornamentation has evolved. From a practical standpoint, there’s something kind of humorous about small sculptures on the top cornice of a 30 story building. Nobody can appreciate them, unless you’re in a building right across the street near the same floor, and that’s a fairly small audience for the cost. Eventually, building owners realize that they're spending money on style when it can be put somewhere people actually experience it. That’s a major reason why there’s a predominance of large buildings that have a very ‘clean’ design. Post modernism took a swing at that, but with large and cost effective ways to build ornamental gestures.
. (C) Brant Fetter. Quora