Being charitable, the thinking at the time was somewhat mechanistic: that you could build utopic urban environments from first principles as massive campuses vs individual buildings.
Inevitable these projects underestimated the organic and evolutionary complexity of urban environments. No single design team organized by some maestro can come close to anticipating and designing around all the needs of an urban neighborhood.
But on the other hand, these mega projects were a simple story to sell politically. Look, we're being bold and thinking big, we're going to actually address problems in housing supply vs kicking the can down the road, etc. Someone else saying they needed to think in terms of flexible zoning and a variety of tax incentives looks not nearly so compelling.
Today we look back on these projects differently, because the failure of "housing projects" in the US, UK, and elsewhere are a familiar meme almost, so people are much more skeptical about mega projects working out.