What made the 1918 Spanish Flu so lethal?

Besides the properties of the virus itself, many additional factors contributed to the virulence of the 1918 pandemic. In 1918, the world was still engaged in World War I. Movement and mobilization of troops placed large numbers of people in close contact and living spaces were overcrowded. Health services were limited, and up to 30% of U.S. physicians were deployed to military service.
In addition, medical technology and countermeasures at the time were limited or non-existent. No diagnostic tests existed at the time that could test for influenza infection. In fact, doctors didn’t know influenza viruses existed. Many health experts at the time thought the 1918 pandemic was caused by a bacterium called “Pfeiffer’s bacillus,” which is now known as Haemophilus influenzae.
Influenza vaccines did not exist at the time, and even antibiotics had not been developed yet. For example, penicillin was not discovered until 1928. Likewise, no flu antiviral drugs were available. Critical care measures, such as intensive care support and mechanical ventilation also were not available in 1918.4 Without these medical countermeasures and treatment capabilities, doctors were left with few treatment options other than supportive care.
In terms of national, state and local pandemic planning, no coordinated pandemic plans existed in 1918. Some cities managed to implement community mitigation measures, such as closing schools, banning public gatherings, and issuing isolation or quarantine orders, but the federal government had no centralized role in helping to plan or initiate these interventions during the 1918 pandemic.

Source: https://www.cdc.gov/flu/pandemic-resources/reconstruction-1918-virus.html