Because people previously (falsely) believed it could not happen in the US, with the last true pandemic being so far in the past. The Bill Gates TED talk from 2

Because people previously (falsely) believed it could not happen in the US, with the last true pandemic being so far in the past. The Bill Gates TED talk from 2016 shows that some people understood what we should have done, but it involves not only changing minds but also putting money into it. Whatever the total fallout from this pandemic, I expect the creation of a medical reserve (as suggested in the talk) is one that will certainly happen.