Basically, like a lot of things that go wrong, it appears to have been down to people who have risen to position of power because they cut corners, and who ignore what Kipling called "the god of the copybook maxims".

In this case the copybook maxim was "Don't put all your eggs in one basket."

The National Health Service in Britain was only planning to get 14 per cent of its needed flu jabs from this company. This meant when it was closed down it was a nuisance but not a crisis.   The USA had intended to get nearly half its supplies from this one firm, and that was nearly half of a far greater number in the first place.

Stupid. And you can guarantee that there will have been some knowledgeable people in the system who warned against this, and whose worries were swept aside. (As with the Iraq war.)