Having to quibble about why it failed means the margins were _way_ too low. It's a semi-permanent structure, if it costs double (and it wont) more to make it redundant in the worst case (loss of multiple main supports for example) then that's a good thing. Skyscrapers for are built with extensive redundancy, hence the pre-weakening needed before bringing one (or 3) down. I bet when the NTSB is done we find out that this bridge wasn't actually up to code.