The reports at the time (of the Taiwan bridge collapse) put the blame on poor maintenance and inspection. There was some criticism of the design in this case but it was not the deciding factor, but as this is from my memory it is probably best if people read the reports themselves and reach their own conclusions.
I think neglect is a major reason, be it due to funding or resource shortages. It's like buying a well-engineered car and not providing the required maintenance.
Or maybe there are times where mistakes are made during construction or unpredictable geological changes?
These issues should be checked case by case. If you have corrupted individuals involved on different levels or some countries with low capabilities to maintain properly then the risk is high?