Because it hasn't, is probably the answer. The Spanish flu, that was probably an American flu, was so devastating for its virulence more than for the fact that it spread. Flu is still a big killer but not to the extent of the 1918 strain.
I’m just wondering why we haven’t seen a lethal strain similiar to spanish flu. All it take is some mutation occuring from chickens and/or pigs where the virus crosses over to humans and kill many more people than 1918 pandemic because the world is much smaller place today than it was 100 years ago.