If you're really looking for causes, the largest one is price optimization.

But, I think one missing piece between architects and the general public is how architecture students are taught to design (and thus, see) buildings first through the lens of massing [0]. When you compose a building primarily by shaping the different masses, you lose sight of the ornamentation and complex decoration that sets old buildings apart. Then, you add air conditioning and modern building systems and the need for high ceilings and windows with airflow also disappears.

What you are left with is a sealed box. Essentially, architecture has ceded too much design space to engineering in the name of cost optimization and there isn't much 'art' left in it. But look around - that's true across our society.

[0] https://en.wikipedia.org/wiki/Massing

Nah I think modern architecture is popular among many architects and unpopular with almost all of the public for the same reason modern art is: it justifies their existence.

It gives them some clever thing to know that normal people don't know anything about. If King Charles is right about architecture (he is), then what new experimental architecture can you research? You can't, because we've basically already discovered what people like.