No one truly has an answer for this one. There are a couple of theories.
First, it killed an unexpected population (the young and healthy), as well as the expected population. This Flu is sometimes referred to as the "one that went for the young".
Second, there was no vaccine. To this day, this flu has been studied and still is little understood.
Finally, another theory claims that people were beginning to travel more, which facilitated the spread of the virus.
I hope this answers your question!