One reason the flu spread so widely (and is a threat today) is the speed of transportation even in 1917. Unlike plague, which has a reservoir and transmission agents in fleas on rats & other rodents, flu requires person to person contact. The "animal" part of flu is from virus mixing of human/animal flu types in birds, pigs, etc. Once a new virulent strain is created transmission is entirely person to person, and the virus has a very short life span once released in to the environment via cough/sneeze.
If you look at the spread of the plague, you'll see how long it took to move across Europe from east to west. Given the relatively small numder of travelers and the speed of transportation (and relative isolation of many spots) during the times of the plague a disease with the transmission specifics of the flu, even if equally fatal, just won't produce the same results. One of the reasons the flu was so effective, in addition to its genetics, was the movement of large numbers of folks all over due to the war, the crowding and poor sanitation etc compared to peacetime early 20th century.