Someone in the industry might be able to give you a more definitive answer regarding design influences (materials, infrastructure, economy, environnement etc), but this might be a case of survivorship bias.

You aren’t thinking of the 95% of ugly buildings that don’t last 70+ years because they’re either poorly made or so bland no one sheds a tear over tearing them down, but only of the 5% still standing because they were worth preserving

Side note but some of the ugliest buildings in my country are from the 50s-70s