I'm confused: what didn't work in Seattle/SF/Portland?

Enforcement didn't work because people won't follow the law anyway or engineering didn't work because people tried to drive through the obstacles or approach them with the same speed and smashed/smooshed more?

Engineering didn't work. Seattle/SF/Portland vigorously attempted to implement the Zero Vision recommendations. The war-on-cars in other words.

And if the problem was in enforcement in the first place, then why do all the engineering that actively worsens the traffic?

SF tried a multi-front approach called "Vision Zero". I think initially deaths went down for a couple years but then ticked back up. No, people aren't (usually) driving through obstacles.