The bridge collapsed some 40 years into a projected 100 year life, because of misguided design choices and greatly increased traffic volume and weight. Designed when cars and trucks were much lighter, using newer and stronger materials than contemporary bridges, but with much less margin and, as it turned out, vulnerable to single point failure. A sober and educational documentary with eyewitness testimony:
Wikipedia says:
… links were composed of only two bars each, of high-strength steel, rather than a thick stack of thinner bars of modest material strength, as is usual. With only two bars, the failure of one could impose excessive loading on the second, causing total failure.
The towers were “rocker” towers [which] required the chain on both sides for their support; failure of any one link on either side, in any of the three chain spans, would result in the complete failure of the entire bridge.
The bridge failure was due to a defect in a single link… A small crack was formed through fretting wear at the bearing, and grew through internal corrosion. The crack was only about 0.1 inches (2.5 mm) deep when it went critical. Growth of the crack was probably exacerbated by residual stress in the eyebar created during manufacture