Sarah Hoyt summarizes:
Not just the conviction that “it can’t happen here.” There is also the deep in-built certainty that tomorrow will be more or less like today, and the worst that can happen within relatively safe bounds. Even while everything is shifting against you. From Pompeii to Nazi Germany, from Alexander’s conquests to Communist Russia, the normalcy bias has killed more human beings than any other factor in history.
See also Note 12 here.