It is partly due to the Influenza Pandemic that killed approximately 20-40 million people worldwide in 1918. It was known as the "Spanish Flu" or "La Grippe", which is where we get the phrase "He died from the Grip". Most of the oldsters going to get theses early flu shots have memory of the Spanish Flu or a family legend about its effects.
During the term of President Gerald Ford, the scare was the "Swine Flu" and the government gave out the shots. A few people died from the shots (millions were given) and the government was sued. The Flu did not materialize as bad as thought.
the advance prediction is that there are mostly mild strains of the flu which will infect the usa this year. The southern hemisphere had a mild year flu wise, and they are usually a leading indicator of our flu season.