There are a couple of reasons modern design won I think.

Art Nouveau and classic styles from before are very expensive to build and maintain. Human population increased massively during the Industrial Revolution leading to more demand for cheap living space, and living inside is more important than looking at it from the outside.

> Art Nouveau and classic styles from before are very expensive to build and maintain.

It's cheaper to do both now than it ever has been.

> living inside is more important than looking at it from the outside.

Even our insides are boring and stupid though. There's no creativity anywhere. It's purely cost. It's just the enshittification of housing. Companies who can cut corners and give you the bare minimum using the cheapest materials can stuff their pockets with the money they save by selling you bland tasteless trash.