The Spanish Flu came on the heels of the Great War.
Many of its victims were already weak and incapacitated as a result of that war. So it was just another of a long line of miseries that the populace had endured. And it burnt itself out within a relatively few months.
Whereas the Great Depression brought an abrupt end to the most prosperous time,to that point, in American history. It wiped out the wealth of countless households and ruined millions of lives. And it went on for over a decade.
Soldiers also ate a poor diet that left their immune systems highly vulnerable.
And also that the Depression was made up of a number of things that took place during Hoover’s and Roosevelt’s administrations. I seem to recall reading Milton Friedman’s account of things and that there seemed to be a mild recovery taking place in 1931 and on into 1932, but the Federal Reserve caused a shortage of cash supply, which created even more bank failures in 1933 and that brought the worst of those years. And also that the NRA (National Recovery Administration) in it’s heavy handed way also added to the trouble.