That plus in the early days traditional/classical architecture was rejected as "bourgeois" or aristocratic by the socialist movements who pushed for modernist approaches. See: the early days of soviet architecture and even Bauhaus in Germany was based on left-leaning/utopian ideas.
So it had dual pressure, from economics because it was cheaper/efficient and from the cultural reform side.
This is sort of a just-so story that isn't completely true. Stalin pushed "socialist classicism" [1] as an aesthetic, and it wasn't until his successors took over that modernist architectural ideas imported from the West became a hallmark of actually-existing socialism.
[1] https://en.wikipedia.org/wiki/Stalinist_architecture