I think it was just sheer incompetence. If you look at subreddits like /r/urbanplanning, the mistakes are clearly recognized and modern urban planners are desperately trying to undo their predecessors' mistakes, but the hole has been dug, and these things take time.

Wouldn’t people on that subreddit be similar to people on HN: fringe and rather ahead of the curve (relatively speaking)?

I don’t know much about urban planning but I see it in tech: the stuff discussed on HN and presented as more or less normal/best practice isn’t being done or even thought of as “best practice” outside of the HN crowd. See enterprise development and a vast number of agencies or other companies that, together, make the bulk of the software being made and sold today where basic things like unit testing or the open/closed principle are seen as a waste of time and unnecessary.