It's not necessarily that commercial activity was rejected - it was often accepted but only in spaces designated by the designers, which in turn bore no resemblance to where people actually wanted to shop, eat, entertain, etc. And so businesses failed to populate these spaces, and they became wastelands, with crime and decay following shortly after.

This philosophy extended beyond commercial spaces - these communities inevitably had spaces set aside for children to play, adults to entertain, etc, and deeply restricted the use of these spaces to these things, and these things only. There's a deep, controlling, Big Brother-esque undercurrent to that entire branch of urban planning.

And through all of this those same urban planners seemed to blame residents to failing to uphold their utopian urban vision than realize that a reasonable bit of laissez-faire is fundamental to functional cities.