I think part of it comes down to the fact that a lot of these places were never urban - they may have had big buildings but the people never experienced street life in the way older American cities - and most European cities - have experienced it (see: Boston, NYC). As a result urban planning is too much an exercise in theory and too little based on pragmatic experience.
Another component is that Radiant City-esque designs look better on paper. You have tall, majestic buildings where architects can go wild without the constraint of adjacent buildings, and ample parkland appeals to our intuitive sense of what must be pleasant, but isn't in reality. In areas where urban decay has been especially problematic, these designs afford residents a sense of safety - they are safely ensconced in towers of glass and steel and may enter and exit with their cars. In many American cities avoidance of the street is sadly still a feature, not a bug.
From what I've read, that is the case for Brasilia. Looks great on paper. They got to start from scratch in the middle of the jungle, kind of like Washington DC when L'enfant had similar blank slate. In any event, while Brasilia looked good and grandiose on paper, it was nowhere near human scale.
It actually works surprisingly well in the residential "superblocks" because the apartment buildings are all raised on pillars so you can walk in a straight line between any two arbitrary points within a superblock, there are plenty of trees and parks, and the local commercial zones are easily accessibly by foot. But then when you get to the center with the bus station and government buildings it's a mass of huge roads and wide open spaces choked with exhaust fumes that's very unpleasant to walk around.
I think part of it comes down to the fact that a lot of these places were never urban - they may have had big buildings but the people never experienced street life in the way older American cities - and most European cities - have experienced it (see: Boston, NYC). As a result urban planning is too much an exercise in theory and too little based on pragmatic experience.
Another component is that Radiant City-esque designs look better on paper. You have tall, majestic buildings where architects can go wild without the constraint of adjacent buildings, and ample parkland appeals to our intuitive sense of what must be pleasant, but isn't in reality. In areas where urban decay has been especially problematic, these designs afford residents a sense of safety - they are safely ensconced in towers of glass and steel and may enter and exit with their cars. In many American cities avoidance of the street is sadly still a feature, not a bug.