Why Modern Art
The public has apparently consistently disliked modernist artistic styles in many areas, including architecture:
Americans prefer traditional/classical buildings to modern ones by about 70% to 30% (regardless of political affiliation!). In a poll of America’s favorite architecture, 76% of buildings selected were traditional/classical. A study of courthouse architecture determined that … ‘most non-architects dislike ‘modern’ design and have done so for almost a century.’ Yet 92% of new federal government buildings are modern. (More)
Which raises the question of why such styles are so often chosen. The obvious proximate answer is that elites choose them to impress other elites, who tend to prefer specialist not popular evaluations, even when they represent democratic governments. But why do they do this?
Scott Alexander, author of the above quote, suggests as explanations: elites trying to hide not flaunt wealth, getting out of touch with popular tastes, becoming more Protestant, seeing timeless aesthetic truths, responding to rising labor costs, splitting off from popular tastes, or signaling taste instead of wealth. Samuel Hughes says:
Modernism was driven by artists & the intellectual circles that surrounded them. In the case of some arts, … music … literature, it never really left those circles: … In … other art forms, rich people were eventually drawn in to some degree. (more)
Which seems right, but raises the question of why artists diverged and how they were able to get away with it.
When search for explanations of social trends, it seems better if possible to focus on trends which one well understands in fundamental terms, so one needn’t ask what caused those trends. And one such key trend for understand the modern world is: the rise of professions.
Medieval guilds declined greatly in power after ~1600. Since ~1800 one’s profession has become a more central part of one’s status and identity, as religion, ethnic, and national identities have faded. The rise of work knowledge and specialization, and of living density, made it harder to pick professionals using a local network and reputation, and so people became more accepting of professions gaining more autonomy to control the quality of their members.
For example, from 1800 to 1950, academics wrested control from outsiders via replacing prices with grants, giving professors tenure, and using peer review for evaluations. Doctors gained the power to control malpractice liability, medical schools, and professional licensing. Judges prevented legislators from regulating the practice of law, so law controlled its own rules and licensing. Clergy have long had such autonomy, and engineers gained quite a lot.
The same modern attitude of indulgence toward professions seeking to judge and control their own quality also made people more willing to defer to artists on artistic quality. This supported a move from individual patrons to salons, galleries, museums listening to curators, critics, and journals.
So the move where artists successfully pushed modernism even when most customers didn’t care for it would just not have been possible a century or more before it happened. Of course this doesn’t explain the more precise timing and direction of the modernist change, which I’ve suggested is partly a response to recent social mixing of different cultures undermining tradition, the rise of abstraction encouraging that in art, and the rise of high school inducing the rise of activist youth movements.


Rockefeller and their ilk started MOMA .
Massive money laundering through ugly art.
The communist style architecture posing as Green.
Ugly Art
Nothing inspired.
Soulless
All to dull the crowds into Grey. The popular color these days. Grey.
I'm surprised you didn't link to your colleague (and old co-author) Tyler Cowen's "An Economic Theory of Avant-Garde and Popular Art, or High and Low Culture" https://gwern.net/doc/culture/2000-cowen.pdf